extensions: Cleanup class/struct fwd declarations

extensions/browser/event_page_tracker.h

 // Copyright 2015 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_BROWSER_EVENT_PAGE_TRACKER_H_
 #define EXTENSIONS_BROWSER_EVENT_PAGE_TRACKER_H_
 
 #include <string>
 
 #include "base/callback.h"
 
 namespace extensions {
 
-class Extension;
-class ExtensionHost;
-
 // Tracks an extension's event page suspend state.
 class EventPageTracker {
  public:
   // Returns true if an extension's event page is active,
   // or false if it is suspended.
   virtual bool IsEventPageSuspended(const std::string& extension_id) = 0;
 
   // Wakes an extension's event page from a suspended state and calls
   // |callback| after it is reactivated.
   //
   // |callback| will be passed true if the extension was reactivated
   // successfully, or false if an error occurred.
   //
   // Returns true if a wake operation was scheduled successfully,
   // or false if the event page was already awake.
   // Callback will be run asynchronously if true, and never run if false.
   virtual bool WakeEventPage(const std::string& extension_id,
                              const base::Callback<void(bool)>& callback) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EVENT_PAGE_TRACKER_H_