Use BlobCallback in toBlob instead of FileCallback

third_party/WebKit/LayoutTests/fast/canvas/canvas-toBlob-file-vs-blob.html

+<script src = "../../resources/js-test.js"></script>
+<script type = "text/javascript"> 
+if (window.testRunner)
+{
+    testRunner.dumpAsText();
+    testRunner.waitUntilDone();
+}
+
+description("Test if toBlob creates a Blob-type object.");
+
+var resultImageObject = null;
+canvas = document.createElement('canvas');
+canvas.toBlob(function(b) {
+    resultImageObject = b;
+    shouldBeFalse('resultImageObject instanceof File');
+    shouldBeTrue('resultImageObject instanceof Blob');
+    if (window.testRunner)
+        testRunner.notifyDone();
+});
+</Script>