Make RootScroller set the outer viewport scroll layer in the compositor

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 189 matching lines @@
   LayerImpl* CurrentlyScrollingLayer() const;
   int LastScrolledLayerId() const;
   void SetCurrentlyScrollingLayer(LayerImpl* layer);
   void ClearCurrentlyScrollingLayer();
 
   void SetViewportLayersFromIds(int overscroll_elasticity_layer,
                                 int page_scale_layer_id,
                                 int inner_viewport_scroll_layer_id,
                                 int outer_viewport_scroll_layer_id);
   void ClearViewportLayers();
+
+  void SetRootScrollerLayerFromId(int root_scroller_layer_id);
+  LayerImpl* RootScrollerLayer() { return LayerById(root_scroller_layer_id_); }
+
   LayerImpl* OverscrollElasticityLayer() {
     return LayerById(overscroll_elasticity_layer_id_);
   }
   LayerImpl* PageScaleLayer() { return LayerById(page_scale_layer_id_); }
   void ApplySentScrollAndScaleDeltasFromAbortedCommit();
 
   SkColor background_color() const { return background_color_; }
   void set_background_color(SkColor color) { background_color_ = color; }
 
   bool has_transparent_background() const {
@@ skipping 287 matching lines @@
   HeadsUpDisplayLayerImpl* hud_layer_;
   PropertyTrees property_trees_;
   SkColor background_color_;
   bool has_transparent_background_;
 
   int last_scrolled_layer_id_;
   int overscroll_elasticity_layer_id_;
   int page_scale_layer_id_;
   int inner_viewport_scroll_layer_id_;
   int outer_viewport_scroll_layer_id_;
+  int root_scroller_layer_id_;
 
   LayerSelection selection_;
 
   scoped_refptr<SyncedProperty<ScaleGroup>> page_scale_factor_;
   float min_page_scale_factor_;
   float max_page_scale_factor_;
 
   float device_scale_factor_;
   float painted_device_scale_factor_;
 
@@ skipping 61 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_