Clean up forward declarations in Source/core/html

Source/core/html/canvas/WebGLContextGroup.h

 /*
  * Copyright (C) 2011 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 19 matching lines @@
 #include "wtf/HashSet.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefCounted.h"
 
 namespace blink {
 class WebGraphicsContext3D;
 }
 
 namespace blink {
 
-class WebGLExtension;
 class WebGLSharedObject;
 class WebGLRenderingContextBase;
 
 typedef int ExceptionCode;
 
 class WebGLContextGroup FINAL : public RefCounted<WebGLContextGroup> {
 public:
     static PassRefPtr<WebGLContextGroup> create();
     ~WebGLContextGroup();
 
@@ skipping 20 matching lines @@
     // registration with WebGLContextGroup, and vice versa, the
     // WebGLContextGroup takes care of detaching the group objects if
     // the set of WebGLRenderingContextBase contexts becomes empty.
     HashSet<WebGLRenderingContextBase*> m_contexts;
     HashSet<WebGLSharedObject*> m_groupObjects;
 };
 
 } // namespace blink
 
 #endif // WebGLContextGroup_h