Add a new API for testing animated images

Source/platform/graphics/Image.h

Index: Source/platform/graphics/Image.h
diff --git a/Source/platform/graphics/Image.h b/Source/platform/graphics/Image.h
index 641ef762a4b1e65de8127e7d0908725bc5f0424e..a019a07e9b58ce6d86a1f521bf1effd280a812f0 100644
--- a/Source/platform/graphics/Image.h
+++ b/Source/platform/graphics/Image.h
@@ -121,6 +121,11 @@ public:
     virtual ImageAnimationPolicy animationPolicy() { return ImageAnimationPolicyAllowed; }
     virtual void advanceTime(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.
+    virtual void advanceAnimationForTesting() { }
+
     // Typically the ImageResource that owns us.
     ImageObserver* imageObserver() const { return m_imageObserver; }
     void setImageObserver(ImageObserver* observer) { m_imageObserver = observer; }