[HTMLCanvasElement.toBlob] Default callback version without scheduler

LayoutTests/fast/canvas/canvas-toBlob-webp-maximum-quality.html

+<canvas id="mycanvas"></canvas>
+<img id="result" onload="testDone()">
+<script type = 'text/javascript'>
+if (window.testRunner) {
+    testRunner.dumpAsTextWithPixelResults();
+    testRunner.waitUntilDone();
+}
+
+function testDone()
+{
+    if (window.testRunner)
+        testRunner.notifyDone();
+}
+
+var image = new Image();
+image.onload = function() {
+        var canvas = document.getElementById("mycanvas");
+        canvas.width = image.width;
+        canvas.height = image.height;
+        var ctx = canvas.getContext('2d');
+        ctx.drawImage(image, 0, 0);
+
+        canvas.toBlob(function(blob) {
+            url = URL.createObjectURL(blob);
+            result.src = url;
+        }, "image/webp", 1.0);

[Justin Novosad, 2015/08/20 20:38:43]

No need to solve this now, but please log a bug for it: the visual result from
this test are disappointing. Maximum quality compression should be lossless or
near lossless. The current results are visibly blurry.

[xlai (Olivia), 2015/08/20 21:36:29]

Done. Issued Logged: https://code.google.com/p/chromium/issues/detail?id=523098

May I ask if this is related to Issue 170565 (the alpha channel webp one) as
well?

+}
+image.src = "resources/letters.png";
+</script>