Refactor prepareToMeasureValuesAsync to startMeasureValuesAsyn which run test through callback

third_party/WebKit/PerformanceTests/Canvas/toBlob_duration.html

Index: third_party/WebKit/PerformanceTests/Canvas/toBlob_duration.html
diff --git a/third_party/WebKit/PerformanceTests/Canvas/toBlob_duration.html b/third_party/WebKit/PerformanceTests/Canvas/toBlob_duration.html
index 9be0c9120108a044f761771a1d64401f6e94f5d6..c19b38d0b38986e483eefeaa7c0698ca5a87b44d 100644
--- a/third_party/WebKit/PerformanceTests/Canvas/toBlob_duration.html
+++ b/third_party/WebKit/PerformanceTests/Canvas/toBlob_duration.html
@@ -44,9 +44,11 @@ window.onload = function () {
         done: function () {
             isDone = true;
         },
-        description: "Measures performance of canvas." 
+        run: function() {
+            runTest(canvas_idle);
+        },
+        description: "Measures performance of canvas."
     });
-    runTest(canvas_idle);
 };
 </script>
 </body>