Refactor page printing logic on the web

- Move the global scratchCanvas to PDFPrintService. This is mainly to
  make it easier to reason about the state of scratchCanvas. In practice
  there is no difference because only one PDFPrintService instance can
  be instantiated at any given time.

- Move all logic of using the rendered page to one location.
  This makes it easier to replace the printing logic later, when I add
  special handling to out-of-process frames in the Chrome extension.

web/pdf_print_service.js

@@ -35,14 +35,10 @@
 
   var activeService = null;
 
-  // Using one canvas for all paint operations -- painting one canvas at a time.
-  var scratchCanvas = null;
-
-  function renderPage(pdfDocument, pageNumber, size, wrapper) {
-    var activeServiceOnEntry = activeService;
-    if (!scratchCanvas) {
-      scratchCanvas = document.createElement('canvas');
-    }
+  // Renders the page to the canvas of the given print service, and returns
+  // the suggested dimensions of the output page.
+  function renderPage(activeServiceOnEntry, pdfDocument, pageNumber, size) {
+    var scratchCanvas = activeService.scratchCanvas;
 
     // The size of the canvas in pixels for printing.
     var PRINT_RESOLUTION = 150;
@@ -51,9 +47,8 @@
     scratchCanvas.height = Math.floor(size.height * PRINT_UNITS);
 
     // The physical size of the img as specified by the PDF document.
-    var img = document.createElement('img');
-    img.style.width = Math.floor(size.width * CSS_UNITS) + 'px';
-    img.style.height = Math.floor(size.height * CSS_UNITS) + 'px';
+    var width = Math.floor(size.width * CSS_UNITS) + 'px';
+    var height = Math.floor(size.height * CSS_UNITS) + 'px';
 
     var ctx = scratchCanvas.getContext('2d');
     ctx.save();
@@ -70,20 +65,10 @@
       };
       return pdfPage.render(renderContext).promise;
     }).then(function () {
-      activeServiceOnEntry.throwIfInactive();
-      if (('toBlob' in scratchCanvas) &&
-          !pdfjsLib.PDFJS.disableCreateObjectURL) {
-        scratchCanvas.toBlob(function (blob) {
-          img.src = URL.createObjectURL(blob);
-        });
-      } else {
-        img.src = scratchCanvas.toDataURL();
-      }
-      wrapper.appendChild(img);
-      return new Promise(function(resolve, reject) {
-        img.onload = resolve;
-        img.onerror = reject;
-      });
+      return {
+        width: width,
+        height: height,
+      };
     });
   }
 
@@ -91,8 +76,9 @@
     this.pdfDocument = pdfDocument;
     this.pagesOverview = pagesOverview;
     this.printContainer = printContainer;
-    this.wrappers = [];
     this.currentPage = -1;
+    // The temporary canvas where renderPage paints one page at a time.
+    this.scratchCanvas = document.createElement('canvas');
   }
 
   PDFPrintService.prototype = {
@@ -100,7 +86,6 @@
       this.throwIfInactive();
 
       var pdfDocument = this.pdfDocument;
-      var printContainer = this.printContainer;
       var body = document.querySelector('body');
       body.setAttribute('data-pdfjsprinting', true);
 
@@ -131,12 +116,6 @@
         '@page { size: ' + pageSize.width + 'pt ' + pageSize.height + 'pt;}' +
         '}';
       body.appendChild(this.pageStyleSheet);
-
-      for (var i = 0, ii = this.pagesOverview.length; i < ii; ++i) {
-        var wrapper = document.createElement('div');
-        printContainer.appendChild(wrapper);
-        this.wrappers[i] = wrapper;
-      }
     },
 
     destroy: function () {
@@ -147,16 +126,13 @@
         return;
       }
       this.printContainer.textContent = '';
-      this.wrappers = null;
       if (this.pageStyleSheet && this.pageStyleSheet.parentNode) {
         this.pageStyleSheet.parentNode.removeChild(this.pageStyleSheet);
         this.pageStyleSheet = null;
       }
+      this.scratchCanvas.width = this.scratchCanvas.height = 0;
+      this.scratchCanvas = null;
       activeService = null;
-      if (scratchCanvas) {
-        scratchCanvas.width = scratchCanvas.height = 0;
-        scratchCanvas = null;
-      }
       ensureOverlay().then(function () {
         if (OverlayManager.active !== 'printServiceOverlay') {
           return; // overlay was already closed
@@ -176,13 +152,41 @@
         }
         var index = this.currentPage;
         renderProgress(index, pageCount);
-        renderPage(this.pdfDocument, index + 1,
-                   this.pagesOverview[index], this.wrappers[index]).then(
-          function () { renderNextPage(resolve, reject); }, reject);
+        renderPage(this, this.pdfDocument, index + 1, this.pagesOverview[index])
+          .then(this.useRenderedPage.bind(this))
+          .then(function () {
+            renderNextPage(resolve, reject);
+          }, reject);
       }.bind(this);
       return new Promise(renderNextPage);
     },
 
+    useRenderedPage: function (printItem) {
+      this.throwIfInactive();
+      var img = document.createElement('img');
+      img.style.width = printItem.width;
+      img.style.height = printItem.height;
+
+      var scratchCanvas = this.scratchCanvas;
+      if (('toBlob' in scratchCanvas) &&
+          !pdfjsLib.PDFJS.disableCreateObjectURL) {
+        scratchCanvas.toBlob(function (blob) {
+          img.src = URL.createObjectURL(blob);
+        });
+      } else {
+        img.src = scratchCanvas.toDataURL();
+      }
+
+      var wrapper = document.createElement('div');
+      wrapper.appendChild(img);
+      this.printContainer.appendChild(wrapper);
+
+      return new Promise(function (resolve, reject) {
+        img.onload = resolve;
+        img.onerror = reject;
+      });
+    },
+
     get active() {
       return this === activeService;
     },