extensions: Cleanup class/struct fwd declarations

extensions/common/permissions/manifest_permission.h

 // Copyright 2013 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_COMMON_PERMISSIONS_MANIFEST_PERMISSION_H_
 #define EXTENSIONS_COMMON_PERMISSIONS_MANIFEST_PERMISSION_H_
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/pickle.h"
 #include "extensions/common/permissions/api_permission_set.h"
 
 namespace base {
 class PickleIterator;
 class Value;
 }
 
-namespace IPC {
-class Message;
-}
-
 namespace extensions {
 
 // Represent the custom behavior of a top-level manifest entry contributing to
 // permission messages and storage.
 class ManifestPermission {
  public:
   ManifestPermission();
   virtual ~ManifestPermission();
 
   // The manifest key this permission applies to.
@@ skipping 46 matching lines @@
   // Logs this permission.
   void Log(std::string* log) const;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ManifestPermission);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_PERMISSIONS_MANIFEST_PERMISSION_H_