Ensure that PDFHistory does not incorrectly replace the initial document hash (issue 6079)

This is fallout from PR 5971; fixes 6079.

web/viewer.js

@@ -799,9 +799,13 @@ var PDFViewerApplication = {
           window.history.replaceState(null, '');
         }
         self.pdfHistory.initialize(self.documentFingerprint);
+
+        if (self.pdfHistory.initialDestination) {
           self.initialDestination = self.pdfHistory.initialDestination;
+        } else if (self.pdfHistory.initialBookmark) {
           self.initialBookmark = self.pdfHistory.initialBookmark;
         }
+      }
 
       store.initializedPromise.then(function resolved() {
         var storedHash = null;