Cleanup: Remove some unneeded Extension* class forward declarations.

chrome/browser/extensions/extension_garbage_collector.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 CHROME_BROWSER_EXTENSIONS_EXTENSION_GARBAGE_COLLECTOR_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_GARBAGE_COLLECTOR_H_
 
 #include <map>
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/extensions/install_observer.h"
 #include "components/keyed_service/core/keyed_service.h"
 
 namespace content {
 class BrowserContext;
 }
 
-class ExtensionService;
-
 namespace extensions {
 
 // The class responsible for cleaning up the cruft left behind on the file
 // system by uninstalled (or failed install) extensions.
 // The class is owned by ExtensionService, but is mostly independent. Tasks to
 // garbage collect extensions and isolated storage are posted once the
 // ExtensionSystem signals ready.
 class ExtensionGarbageCollector : public KeyedService, public InstallObserver {
  public:
   explicit ExtensionGarbageCollector(content::BrowserContext* context);
@@ skipping 51 matching lines @@
   // Generate weak pointers for safely posting to the file thread for garbage
   // collection.
   base::WeakPtrFactory<ExtensionGarbageCollector> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionGarbageCollector);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_GARBAGE_COLLECTOR_H_