ScrollAnimatorMac should post contentAreaScrolled only for explicit scrolls.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 9ff924298d18d1da1576dc6575c0a991110cc341..d17602cb1561833fc7adc76b38648b3f4074b35e 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1976,7 +1976,8 @@ void FrameView::DidUpdateElasticOverscroll() {
         elastic_overscroll.Width() - HorizontalScrollbar()->ElasticOverscroll();
     if (delta != 0) {
       HorizontalScrollbar()->SetElasticOverscroll(elastic_overscroll.Width());
-      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(delta, 0));
+      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(delta, 0),
+                                                    kCompositorScroll);
       SetScrollbarNeedsPaintInvalidation(kHorizontalScrollbar);
     }
   }
@@ -1985,7 +1986,8 @@ void FrameView::DidUpdateElasticOverscroll() {
         elastic_overscroll.Height() - VerticalScrollbar()->ElasticOverscroll();
     if (delta != 0) {
       VerticalScrollbar()->SetElasticOverscroll(elastic_overscroll.Height());
-      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(0, delta));
+      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(0, delta),
+                                                    kCompositorScroll);
       SetScrollbarNeedsPaintInvalidation(kVerticalScrollbar);
     }
   }