Add a new API for testing animated images

Source/core/testing/Internals.idl

Index: Source/core/testing/Internals.idl
diff --git a/Source/core/testing/Internals.idl b/Source/core/testing/Internals.idl
index 040bd7d78a209a3e1b57702f88ab0040924a2349..e85d56ee548e859e7fe75f1665c900d9c195f1dc 100644
--- a/Source/core/testing/Internals.idl
+++ b/Source/core/testing/Internals.idl
@@ -78,6 +78,11 @@
     // for testing whether animated images work properly.
     [RaisesException] void advanceTimeForImage(Element image, double deltaTimeInSeconds);
 
+    // Advances an animated image. For BitmapImage (e.g., animated gifs) this
+    // will advance to the next frame. For SVGImage, this will trigger an
+    // animation update for CSS and advance the SMIL timeline by one frame.
+    [RaisesException, TypeChecking=Interface] void advanceImageAnimation(Element image);
+
     [RaisesException, TypeChecking=Interface] Node nextSiblingInComposedTree(Node node);
     [RaisesException, TypeChecking=Interface] Node firstChildInComposedTree(Node node);
     [RaisesException, TypeChecking=Interface] Node lastChildInComposedTree(Node node);