Make scrollAnimatorEnabled work on Mac like it does on other platforms

ui/gfx/animation/animation.h

Index: ui/gfx/animation/animation.h
diff --git a/ui/gfx/animation/animation.h b/ui/gfx/animation/animation.h
index 44823b0366f1ae220835a9c97ab1d3ebdca0b701..235bdb3537fa2e90bc64a6fe443dcffb079701b9 100644
--- a/ui/gfx/animation/animation.h
+++ b/ui/gfx/animation/animation.h
@@ -64,6 +64,10 @@ class GFX_EXPORT Animation : public AnimationContainerElement {
   // to give guidance for heavy animations such as "start download" arrow.
   static bool ShouldRenderRichAnimation();
 
+  // Determines on a per-platform basis whether scroll animations
+  // (e.g. produced by home/end key) should be enabled.
+  static bool ScrollAnimationsEnabledBySystem();
+
  protected:
   // Invoked from Start to allow subclasses to prepare for the animation.
   virtual void AnimationStarted() {}