[HTMLCanvasElement.toBlob] Default callback version without scheduler

LayoutTests/fast/canvas/canvas-toBlob-defaultpng.html

+<img id="myImg" onload="testDone()">
+<script src = "../../resources/js-test.js"></script>

[xlai (Olivia), 2015/08/21 18:40:05]

Using asynchronous testing from js-test testing framework instead of testharness
framework. This is now a pixel test (directly comparing end result from toBlob).

+<script type = 'text/javascript'>
+if (window.testRunner) {
+    testRunner.dumpAsTextWithPixelResults();

[Justin Novosad, 2015/08/21 18:53:43]

This should not be a pixel test. We only use pixel tests as a last resort, in
case where text or ref baselines cannot get the job done.  This is because image
baselines are a burden to maintain and they take a lot of space in the repo. 
The other tests you create are appropriate as pixel tests because the baselines
will capture subtleties in compression quality/artifacts.

+    testRunner.waitUntilDone();
+}
+
+function testDone() {
+    if (window.testRunner)
+        testRunner.notifyDone();
+}
+
+var newImg = document.getElementById("myImg");
+var canvas = document.createElement("canvas");
+var ctx = canvas.getContext("2d");
+ctx.fillStyle = "#FF0000";
+ctx.fillRect(0, 0, 150, 75);
+
+canvas.toBlob(function(blob) {
+    url = URL.createObjectURL(blob);
+    newImg.src = url;
+});
+</script>
+</head>
+</body>
+</html>