Debounces scroll events in web viewer.

As requested in #5178, this change debounces the scroll events.
The reason for doing so is that browsers can event-storm especially on
scroll, communicating hundreds of subpixel changes.

The main reason for this resulting in poor performance is that on each
scroll event `scrollTop` was queried, which forces layouting.

This change will use `requestAnimationFrame` to make sure the browser can
allocate enough time to other tasks. The delay is however quite small, thus
the reduction in executions is less noticeable. Modern browsers however utilize
`requestAnimationFrame` to smoothen out rendering.

web/viewer.js

@@ -274,7 +274,14 @@ var PDFView = {
   watchScroll: function pdfViewWatchScroll(viewAreaElement, state, callback) {
     state.down = true;
     state.lastY = viewAreaElement.scrollTop;
-    viewAreaElement.addEventListener('scroll', function webViewerScroll(evt) {
+    state.rAF = null;
+    viewAreaElement.addEventListener('scroll', function debounceScroll(evt) {
+      if (state.rAF) {
+        return;
+      }
+      // schedule an invocation of webViewerScrolled for next animation frame.
+      state.rAF = window.requestAnimationFrame(function webViewerScrolled() {
+        state.rAF = null;
         if (!PDFView.pdfDocument) {
           return;
         }
@@ -288,6 +295,7 @@ var PDFView = {
         // else do nothing and use previous value
         state.lastY = currentY;
         callback();
+      });
     }, true);
   },