Revert of Simplify CanvasAsyncBlobCreator by removing ContextObserver and related tests

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

+<script src = "../../../resources/js-test.js"></script>
+<script type = 'text/javascript'>
+jsTestIsAsync = true;
+description("Test that verifies whether the image data survives the toBlob process after async image encoding");
+
+if (window.testRunner) {
+    testRunner.dumpAsText();
+    testRunner.waitUntilDone();
+}
+
+var canvas = document.createElement("canvas");
+var ctx = canvas.getContext("2d");
+ctx.fillStyle = "#FF0000";
+ctx.fillRect(0, 0, 150, 75);
+var canvas2 = document.createElement("canvas");
+var ctx2 = canvas2.getContext("2d");
+
+var newImg = new Image();
+newImg.onload = function() {
+    // 300x150 is the default size of the canvas, which is the source of the newImg.
+    ctx2.drawImage(newImg, 0, 0, 300, 150);
+
+    var imageData1 = ctx.getImageData(0, 0, 150, 75).data;
+    var imageData2 = ctx2.getImageData(0, 0, 150, 75).data;
+    var imageMatched = true;
+    for (var i = 1; i < imageData1.length; i++)
+    {
+        if (imageData1[i]!=imageData2[i])
+        {
+            imageMatched = false;
+            break;
+        }
+    }
+    if (imageMatched)
+        testPassed("image data survives through the toBlob and PNG Image encoder");
+    else 
+        testFailed("image data does not survive through the toBlob and PNG Image encoder");
+
+    finishJSTest();
+}
+
+canvas.toBlob(function(blob) {
+    url = URL.createObjectURL(blob);
+    newImg.src = url;
+});
+
+</script>