extensions: Cleanup class/struct fwd declarations

extensions/common/permissions/manifest_permission_set.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_SET_H_
 #define EXTENSIONS_COMMON_PERMISSIONS_MANIFEST_PERMISSION_SET_H_
 
 #include <string>
 #include <vector>
 
 #include "base/strings/string16.h"
 #include "extensions/common/permissions/base_set_operators.h"
 
 namespace base {
 class ListValue;
 }  // namespace base
 
 namespace extensions {
 
-class Extension;
 class ManifestPermission;
 class ManifestPermissionSet;
 
 template<>
 struct BaseSetOperatorsTraits<ManifestPermissionSet> {
   typedef ManifestPermission ElementType;
   typedef std::string ElementIDType;
 };
 
 class ManifestPermissionSet : public BaseSetOperators<ManifestPermissionSet> {
  public:
   // Parses permissions from |permissions| and adds the parsed permissions to
   // |manifest_permissions|. If |unhandled_permissions| is not NULL, the names
   // of all permissions that couldn't be parsed will be added to this vector.
   // If |error| is NULL, parsing will continue with the next permission if
   // invalid data is detected. If |error| is not NULL, it will be set to an
   // error message and false is returned when an invalid permission is found.
   static bool ParseFromJSON(
       const base::ListValue* permissions,
       ManifestPermissionSet* manifest_permissions,
       base::string16* error,
       std::vector<std::string>* unhandled_permissions);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_PERMISSIONS_MANIFEST_PERMISSION_SET_H_