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

cc/trees/layer_tree_impl.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 447 matching lines @@
         event_properties;
   }
 
   void ResetAllChangeTracking();
 
   void AddToLayerList(LayerImpl* layer);
 
   void ClearLayerList();
 
   void BuildLayerListForTesting();
+  void ShowScrollbars();
 
   void InvalidateRegionForImages(const ImageIdFlatSet& images_to_invalidate);
 
  protected:
   float ClampPageScaleFactorToLimits(float page_scale_factor) const;
   void PushPageScaleFactorAndLimits(const float* page_scale_factor,
                                     float min_page_scale_factor,
                                     float max_page_scale_factor);
   bool SetPageScaleFactorLimits(float min_page_scale_factor,
                                 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();
 
   LayerTreeHostImpl* layer_tree_host_impl_;
   int source_frame_number_;
   int is_first_frame_after_commit_tracker_;
   LayerImpl* root_layer_for_testing_;
   HeadsUpDisplayLayerImpl* hud_layer_;
   PropertyTrees property_trees_;
@@ skipping 92 matching lines @@
 
   std::unique_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_