Add a new API for testing animated images

Source/platform/graphics/BitmapImage.h

Index: Source/platform/graphics/BitmapImage.h
diff --git a/Source/platform/graphics/BitmapImage.h b/Source/platform/graphics/BitmapImage.h
index 821c78b3d2ebf14908983706674a5932b89aad46..61e0594a8072ee8dd424acb9ad668b8ebfd7add7 100644
--- a/Source/platform/graphics/BitmapImage.h
+++ b/Source/platform/graphics/BitmapImage.h
@@ -84,6 +84,9 @@ public:
     ImageAnimationPolicy animationPolicy() override { return m_animationPolicy; }
     void advanceTime(double deltaTimeInSeconds) override;
 
+    // Advance the image animation by one frame.
+    void advanceAnimationForTesting() override { internalAdvanceAnimation(false); }
+
     PassRefPtr<SkImage> imageForCurrentFrame() override;
     PassRefPtr<Image> imageForDefaultFrame() override;
     bool currentFrameKnownToBeOpaque() override;