Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/blink/web_scrollbar_layer_impl.h

 // Copyright 2014 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_BLINK_WEB_SCROLLBAR_LAYER_IMPL_H_
 #define CC_BLINK_WEB_SCROLLBAR_LAYER_IMPL_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "cc/blink/cc_blink_export.h"
 #include "third_party/WebKit/public/platform/WebScrollbar.h"
 #include "third_party/WebKit/public/platform/WebScrollbarLayer.h"
 
 namespace blink {
 class WebScrollbarThemeGeometry;
 class WebScrollbarThemePainter;
 }
 
 namespace cc_blink {
 
 class WebLayerImpl;
 
 class WebScrollbarLayerImpl : public blink::WebScrollbarLayer {
  public:
   CC_BLINK_EXPORT WebScrollbarLayerImpl(
       std::unique_ptr<blink::WebScrollbar> scrollbar,
       blink::WebScrollbarThemePainter painter,
-      std::unique_ptr<blink::WebScrollbarThemeGeometry> geometry);
-  CC_BLINK_EXPORT WebScrollbarLayerImpl(
-      std::unique_ptr<blink::WebScrollbar> scrollbar,
-      blink::WebScrollbarThemePainter painter,
       std::unique_ptr<blink::WebScrollbarThemeGeometry> geometry,
-      bool temp);
+      bool is_overlay,
+      blink::WebLayer* scroll_layer);
   CC_BLINK_EXPORT WebScrollbarLayerImpl(
       blink::WebScrollbar::Orientation orientation,
       int thumb_thickness,
       int track_start,
-      bool is_left_side_vertical_scrollbar);
+      bool is_left_side_vertical_scrollbar,
+      blink::WebLayer* scroll_layer);
   ~WebScrollbarLayerImpl() override;
 
   // blink::WebScrollbarLayer implementation.
   blink::WebLayer* layer() override;
-  void setScrollLayer(blink::WebLayer* layer) override;
 
  private:
   std::unique_ptr<WebLayerImpl> layer_;
 
   DISALLOW_COPY_AND_ASSIGN(WebScrollbarLayerImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_WEB_SCROLLBAR_LAYER_IMPL_H_