Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/painted_scrollbar_layer.h

 // Copyright 2013 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_LAYERS_PAINTED_SCROLLBAR_LAYER_H_
 #define CC_LAYERS_PAINTED_SCROLLBAR_LAYER_H_
 
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 #include "cc/input/scrollbar.h"
@@ skipping 12 matching lines @@
   static scoped_refptr<PaintedScrollbarLayer> Create(
       std::unique_ptr<Scrollbar> scrollbar,
       int scroll_layer_id);
 
   bool OpacityCanAnimateOnImplThread() const override;
   bool AlwaysUseActiveTreeOpacity() const override;
   ScrollbarLayerInterface* ToScrollbarLayer() override;
 
   // ScrollbarLayerInterface
   int ScrollLayerId() const override;
-  void SetScrollLayer(int layer_id) override;
-
   ScrollbarOrientation orientation() const override;
 
   // Layer interface
   bool Update() override;
   void SetLayerTreeHost(LayerTreeHost* host) override;
   void PushPropertiesTo(LayerImpl* layer) override;
 
   const gfx::Size& internal_content_bounds() const {
     return internal_content_bounds_;
   }
@@ skipping 49 matching lines @@
   std::unique_ptr<ScopedUIResource> thumb_resource_;
 
   float thumb_opacity_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintedScrollbarLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_PAINTED_SCROLLBAR_LAYER_H_