Fixes CanvasPixelArray set polyfillfor chrome < 21 (mozilla#4974)

web/compatibility.js

@@ -481,17 +481,35 @@ if (typeof PDFJS === 'undefined') {
   }
 })();
 
-// TODO CanvasPixelArray is deprecated; use Uint8ClampedArray
-// once it's supported.
+// Support: IE<11, Chrome<21
 (function checkSetPresenceInImageData() {
-  if (window.CanvasPixelArray) {
-    if (typeof window.CanvasPixelArray.prototype.set !== 'function') {
-      window.CanvasPixelArray.prototype.set = function(arr) {
+  // because the availability of a set function on ImageData.data is not
+  // feature-detectable, we have to rely on user agents:
+  // IE<11 contains MSIE - starting from 11 it no longer does
+  var polyfill = false;
+  if (navigator.userAgent.indexOf('MSIE') >= 0) {
+    polyfill = true;
+  }
+  // chrome user agent contains Chrome/major.minor
+  if (navigator.userAgent.indexOf('Chrom') >= 0) {
+    var version = navigator.userAgent.match(/Chrom(e|ium)\/([0-9]+)\./)[2];
+    if (parseInt(version) < 21) {
+      polyfill = true;
+    }
+  }
+
+  if (polyfill) {
+    var contextPrototype = window.CanvasRenderingContext2D.prototype;
+    contextPrototype._createImageData = contextPrototype.createImageData;
+    contextPrototype.createImageData = function(w, h) {
+      var imageData = this._createImageData(w, h);
+      imageData.data.set = function(arr) {
         for (var i = 0, ii = this.length; i < ii; i++) {
           this[i] = arr[i];
         }
       };
-    }
+      return imageData;
+    };
   }
 })();