Call SAC::DidScrollUpdate only for compositor-triggered scrolls.

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 7998290d4efe015b55a6d1ff9f51e5fe80b90147..51422ab16181872e04789a291390f10b86eb5831 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -465,6 +465,7 @@ class CC_EXPORT LayerTreeImpl {
   void ClearLayerList();
 
   void BuildLayerListForTesting();
+  void ShowScrollbars();
 
   void InvalidateRegionForImages(const ImageIdFlatSet& images_to_invalidate);
 
@@ -477,7 +478,6 @@ class CC_EXPORT LayerTreeImpl {
                                 float max_page_scale_factor);
   bool IsViewportLayerId(int id) const;
   void UpdateScrollbars(int scroll_layer_id, int clip_layer_id);
-  void ShowScrollbars();
   void DidUpdatePageScale();
   void PushBrowserControls(const float* top_controls_shown_ratio);
   bool ClampBrowserControlsShownRatio();