[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);

[Noel Gordon, 2015/08/28 08:53:47]

image/webp does not have the special treatment that image/jpeg gets at quality
1.0.  Thanks for the bug report.

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