Fix cc scrollbar layer issues with initialization, and use element ids throughout.

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"
@@ skipping 21 matching lines @@
       blink::WebScrollbar::Orientation orientation,
       int thumb_thickness,
       int track_start,
       bool is_left_side_vertical_scrollbar);
   ~WebScrollbarLayerImpl() override;
 
   // blink::WebScrollbarLayer implementation.
   blink::WebLayer* Layer() override;
   void SetScrollLayer(blink::WebLayer* layer) override;
 
+  void SetElementId(const cc::ElementId&) override;

[wkorman, 2017/05/12 22:29:38]

Is it worth using ref param given struct is basically an int? Am fine either
way, I think others have commented similarly to me in past so figured to ask and
learn again from this reviewer set.

+
  private:
   std::unique_ptr<WebLayerImpl> layer_;
 
   DISALLOW_COPY_AND_ASSIGN(WebScrollbarLayerImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_WEB_SCROLLBAR_LAYER_IMPL_H_