Add a new API for testing animated images

LayoutTests/paint/invalidation/animated-gif-background.html

Index: LayoutTests/paint/invalidation/animated-gif-background.html
diff --git a/LayoutTests/paint/invalidation/animated-gif-background.html b/LayoutTests/paint/invalidation/animated-gif-background.html
index 8edb5aed6470b78f7d135724158609c0d4a93eb7..fe5509c482cc73176517a1a0bc9fc82ecca993d2 100644
--- a/LayoutTests/paint/invalidation/animated-gif-background.html
+++ b/LayoutTests/paint/invalidation/animated-gif-background.html
@@ -8,26 +8,15 @@
 <script>
 window.testIsAsync = true;
 
-// Steps:
-// 1. Load the image
-// 2. Start tracking paint invalidation rects
-// 3. Call layoutAndPaintAsyncThen (via runRepaintTest, which does this for us) to draw a frame
-// 4. Wait longer than 100ms, so that the timer between the first and second animated GIF
-//    frame expires and a paint invalidation is issued on the animated GIF. (100ms is the
-//    length of the first frame of the animation in the GIF file.)
-// 5. Stop tracking paint invalidation rects (finishRepaintTest does this for us)
-
-// These steps differ from a standard repaint test in that step 2 precedes step 3.
-// This is critical because layoutAndPaintAsyncThen may take longer than the 100ms of the timer,
-// so we may miss it.
-
 function repaintTest() {
-    setTimeout(finishRepaintTest, 150);
+    window.internals.advanceImageAnimation(testTarget);
+    requestAnimationFrame(function() {
+        finishRepaintTest();
+    });
 }
 
 function targetImageOnload() {
-    if (window.internals)
-        window.internals.startTrackingRepaints(document);
+    window.internals.startTrackingRepaints(document);
     runRepaintTest();
 }