Make DisplayItemClient an interface

third_party/WebKit/Source/platform/exported/WebScrollbarThemeClientImpl.cpp

 /*
  * 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 236 matching lines @@
 bool WebScrollbarThemeClientImpl::thumbNeedsRepaint() const
 {
     return true;
 }
 
 void WebScrollbarThemeClientImpl::setThumbNeedsRepaint(bool)
 {
     ASSERT_NOT_REACHED();
 }
 
-DisplayItemClient WebScrollbarThemeClientImpl::displayItemClient() const
-{
-    ASSERT_NOT_REACHED();
-    return toDisplayItemClient(this);
-}
-
 String WebScrollbarThemeClientImpl::debugName() const
 {
     return "WebScrollbarThemeClientImpl";
 }
 
 } // namespace blink