ScrollAnimatorMac should post contentAreaScrolled only for explicit scrolls.

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 265 matching lines @@
 
   // Tell the scrollbars to update their thumb postions.
   // If the scrollbar does not have its own layer, it must always be
   // invalidated to reflect the new thumb offset, even if the theme did not
   // invalidate any individual part.
   if (Scrollbar* horizontal_scrollbar = this->HorizontalScrollbar())
     horizontal_scrollbar->OffsetDidChange();
   if (Scrollbar* vertical_scrollbar = this->VerticalScrollbar())
     vertical_scrollbar->OffsetDidChange();
 
-  if (GetScrollOffset() != old_offset)
-    GetScrollAnimator().NotifyContentAreaScrolled(GetScrollOffset() -
-                                                  old_offset);
+  if (GetScrollOffset() != old_offset) {
+    GetScrollAnimator().NotifyContentAreaScrolled(
+        GetScrollOffset() - old_offset, scroll_type);
+  }
 
   GetScrollAnimator().SetCurrentOffset(offset);
 }
 
 bool ScrollableArea::ScrollBehaviorFromString(const String& behavior_string,
                                               ScrollBehavior& behavior) {
   if (behavior_string == "auto")
     behavior = kScrollBehaviorAuto;
   else if (behavior_string == "instant")
     behavior = kScrollBehaviorInstant;
@@ skipping 371 matching lines @@
                                          offset.y() - ScrollOrigin().Y());
   SetScrollOffset(new_offset, kCompositorScroll);
 }
 
 DEFINE_TRACE(ScrollableArea) {
   visitor->Trace(scroll_animator_);
   visitor->Trace(programmatic_scroll_animator_);
 }
 
 }  // namespace blink