Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/solid_color_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_SOLID_COLOR_SCROLLBAR_LAYER_H_
 #define CC_LAYERS_SOLID_COLOR_SCROLLBAR_LAYER_H_
 
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 #include "cc/layers/layer.h"
@@ skipping 18 matching lines @@
   bool AlwaysUseActiveTreeOpacity() const override;
   ScrollbarLayerInterface* ToScrollbarLayer() override;
 
   void SetOpacity(float opacity) override;
   void PushPropertiesTo(LayerImpl* layer) override;
 
   void SetNeedsDisplayRect(const gfx::Rect& rect) override;
 
   // ScrollbarLayerInterface
   int ScrollLayerId() const override;
-  void SetScrollLayer(int layer_id) override;
 
   ScrollbarOrientation orientation() const override;
 
   int thumb_thickness() const {
     return solid_color_scrollbar_layer_inputs_.thumb_thickness;
   }
 
   int track_start() const {
     return solid_color_scrollbar_layer_inputs_.track_start;
   }
@@ skipping 30 matching lines @@
   };
 
   SolidColorScrollbarLayerInputs solid_color_scrollbar_layer_inputs_;
 
   DISALLOW_COPY_AND_ASSIGN(SolidColorScrollbarLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SOLID_COLOR_SCROLLBAR_LAYER_H_