ScrollAnimatorMac should post contentAreaScrolled only for explicit scrolls.

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.h

 /*
  * Copyright (C) 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 98 matching lines @@
   void ContentAreaDidShow() const override;
   void ContentAreaDidHide() const override;
 
   void FinishCurrentScrollAnimations() override;
 
   void DidAddVerticalScrollbar(Scrollbar&) override;
   void WillRemoveVerticalScrollbar(Scrollbar&) override;
   void DidAddHorizontalScrollbar(Scrollbar&) override;
   void WillRemoveHorizontalScrollbar(Scrollbar&) override;
 
-  void NotifyContentAreaScrolled(const ScrollOffset& delta) override;
+  void NotifyContentAreaScrolled(const ScrollOffset& delta,
+                                 ScrollType) override;
 
   bool SetScrollbarsVisibleForTesting(bool) override;
 
   ScrollOffset AdjustScrollOffsetIfNecessary(const ScrollOffset&) const;
 
   void ImmediateScrollTo(const ScrollOffset&);
 
   bool have_scrolled_since_page_load_;
   bool needs_scroller_style_update_;
   IntRect visible_scroller_thumb_rect_;
 };
 
 }  // namespace blink
 
 #endif  // ScrollAnimatorMac_h