Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/scrollbar_layer_interface.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_SCROLLBAR_LAYER_INTERFACE_H_
 #define CC_LAYERS_SCROLLBAR_LAYER_INTERFACE_H_
 
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 #include "cc/input/scrollbar.h"
 
 namespace cc {
 
 class CC_EXPORT ScrollbarLayerInterface {
  public:
   virtual int ScrollLayerId() const = 0;
-  virtual void SetScrollLayer(int layer_id) = 0;
 
   virtual ScrollbarOrientation orientation() const = 0;
 
  protected:
   ScrollbarLayerInterface() {}
   virtual ~ScrollbarLayerInterface() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerInterface);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SCROLLBAR_LAYER_INTERFACE_H_