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

third_party/WebKit/public/platform/WebScrollbarLayer.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 12 matching lines @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef WebScrollbarLayer_h
 #define WebScrollbarLayer_h
 
 #include "WebLayer.h"
 #include "WebScrollbarThemeGeometry.h"
 #include "WebScrollbarThemePainter.h"
 
+namespace cc {
+
+struct ElementId;
+}
+
 namespace blink {
 
 class WebScrollbarLayer {
  public:
   virtual ~WebScrollbarLayer() {}
 
   virtual WebLayer* Layer() = 0;
 
   virtual void SetScrollLayer(WebLayer*) = 0;
+
+  // This is an element id for the scrollbar, not the scrolling layer.
+  // This is not to be confused with scrolling_element_id, which is the
+  // element id of the scrolling layer that has a scrollbar.
+  // All scrollbar layers require element ids.
+  virtual void SetElementId(const cc::ElementId&) = 0;
 };
 
 }  // namespace blink
 
 #endif  // WebScrollbarLayer_h