Add a new API for testing animated images

Source/core/testing/Internals.h

Index: Source/core/testing/Internals.h
diff --git a/Source/core/testing/Internals.h b/Source/core/testing/Internals.h
index 476062e07479331ec428d018a98ed9b35adcab6e..c57b90d99d2447ce553a142781e7722149ab4e93 100644
--- a/Source/core/testing/Internals.h
+++ b/Source/core/testing/Internals.h
@@ -116,6 +116,11 @@ public:
     // for testing whether animated images work properly.
     void advanceTimeForImage(Element* image, double deltaTimeInSeconds, ExceptionState&);
 
+    // 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.
+    void advanceImageAnimation(Element* image, ExceptionState&);
+
     bool isValidContentSelect(Element* insertionPoint, ExceptionState&);
     Node* treeScopeRootNode(Node*);
     Node* parentTreeScope(Node*);