Reland Simplify CanvasAsyncBlobCreator by removing ContextObserver and tests

third_party/WebKit/LayoutTests/virtual/threaded/fast/canvas-toBlob/canvas-toBlob-invalid.html

-<script src="../../resources/js-test.js"></script>
+<script src="../../../../resources/js-test.js"></script>
 <script type='text/javascript'>
 description("Test the handling of invalid arguments in canvas toBlob().");
 
+if (window.testRunner) {
+    testRunner.dumpAsText();
+    testRunner.waitUntilDone();
+}
+
+var numAsyncCalls = 2;
+function finishOneAsyncCall()
+{
+    numAsyncCalls--;
+    if (numAsyncCalls == 0 && window.testRunner) {
+        testRunner.notifyDone();
+    }
+}
+
 var canvas = document.createElement('canvas');
 var ctx = canvas.getContext("2d"); 
 ctx.strokeStyle = "red";
 ctx.strokeRect(0, 0, 50, 50);
 
 shouldThrow("canvas.toBlob();");
 shouldThrow("canvas.toBlob(null);");
 shouldThrow("canvas.toBlob(undefined);");
 // Passing the callback argument without blob handle silently fails.
-shouldNotThrow("canvas.toBlob(function() {});");
+shouldNotThrow("canvas.toBlob(function() { finishOneAsyncCall(); });");
 
 // Invalid quality argument will fall back to default value
-shouldNotThrow("canvas.toBlob(function(blob) {}, 'image/jpeg', 500)");
+shouldNotThrow("canvas.toBlob(function(blob) { finishOneAsyncCall(); }, 'image/jpeg', 500)");
 </script>