extensions: Cleanup class/struct fwd declarations

extensions/renderer/lazy_background_page_native_handler.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 EXTENSIONS_RENDERER_LAZY_BACKGROUND_PAGE_NATIVE_HANDLER_H_
 #define EXTENSIONS_RENDERER_LAZY_BACKGROUND_PAGE_NATIVE_HANDLER_H_
 
 #include "extensions/renderer/object_backed_native_handler.h"
 
 namespace extensions {
 
-class Extension;
-
 class LazyBackgroundPageNativeHandler : public ObjectBackedNativeHandler {
  public:
   explicit LazyBackgroundPageNativeHandler(ScriptContext* context);
   void IncrementKeepaliveCount(const v8::FunctionCallbackInfo<v8::Value>& args);
   void DecrementKeepaliveCount(const v8::FunctionCallbackInfo<v8::Value>& args);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_LAZY_BACKGROUND_PAGE_NATIVE_HANDLER_H_