Implement Asynchronous image encoding for Canvas.toBlob

third_party/WebKit/LayoutTests/fast/canvas/canvas-toBlob-toDataURL-race-imageEncoder-webp.html

+<script src = "../../resources/js-test.js"></script>
+<script src = "./script-tests/canvas-toBlob-toDataURL-race.js"></script>
+<script type = 'text/javascript'>
+description("Verifies if WEBP image encoding on main thread (toDataURL) conflicts with image encoding on async thread (toBlob)");
+
+//Fire a bunch of toBlob operations of canvas to keep the async thread busy
+var j = 0; // due to async nature of toBlob we need a separate counter
+for (var i = 0; i < numToBlobCalls; i++) 
+{
+    canvas.toBlob(function(blob) {
+        url = URL.createObjectURL(blob);
+        testImages[j++].src = url;
+    }, "image/webp", 1.0);
+}
+
+//Then file a bunch of toDataURL operation on main thread, so both threads now compete for image encoding
+for (var i = numToBlobCalls; i < (numToDataURLCalls + numToBlobCalls); i++) 
+{
+    testImages[i].src = canvas.toDataURL("image/webp", 1.0);
+}
+</script>