Add willReadFrequently: true to Images test

We expect authors to use this path in the wild so we should prefer
measuring it.

MotionMark1.2/tests/master/resources/image-data.js

@@ -143,7 +143,7 @@ var ImageDataStage = Utilities.createSubclass(Stage,
     animate: function(timeDelta) {
         for (var i = 0; i < this._offsetIndex; ++i) {
             var element = this.testElements[i];
-            var context = element.getContext("2d");
+            var context = element.getContext("2d", { willReadFrequently: true });
 
             // Get image data
             var imageData = context.getImageData(0, 0, this.imageWidth, this.imageHeight);