extensions: Cleanup class/struct fwd declarations

extensions/renderer/app_window_custom_bindings.h

 // Copyright (c) 2012 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_APP_WINDOW_CUSTOM_BINDINGS_H_
 #define EXTENSIONS_RENDERER_APP_WINDOW_CUSTOM_BINDINGS_H_
 
 #include "base/macros.h"
 #include "extensions/renderer/object_backed_native_handler.h"
 
 namespace extensions {
-class ScriptContextSet;
 
 // Implements custom bindings for the app.window API.
 class AppWindowCustomBindings : public ObjectBackedNativeHandler {
  public:
   AppWindowCustomBindings(ScriptContext* context);
 
  private:
   void GetFrame(const v8::FunctionCallbackInfo<v8::Value>& args);
 
   // Return string containing the HTML <template> for the <window-controls>
   // custom element.
   void GetWindowControlsHtmlTemplate(
       const v8::FunctionCallbackInfo<v8::Value>& args);
 
   DISALLOW_COPY_AND_ASSIGN(AppWindowCustomBindings);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_APP_WINDOW_CUSTOM_BINDINGS_H_