chrome: Cleanup class/struct forward declarations

chrome/browser/media/protected_media_identifier_permission_context.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 CHROME_BROWSER_MEDIA_PROTECTED_MEDIA_IDENTIFIER_PERMISSION_CONTEXT_H_
 #define CHROME_BROWSER_MEDIA_PROTECTED_MEDIA_IDENTIFIER_PERMISSION_CONTEXT_H_
 
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "chrome/browser/permissions/permission_context_base.h"
@@ skipping 10 matching lines @@
 class Profile;
 
 namespace views {
 class Widget;
 }
 
 namespace content {
 class WebContents;
 }
 
-namespace user_prefs {
-class PrefRegistrySyncable;
-}
-
 // Manages protected media identifier permissions flow, and delegates UI
 // handling via PermissionQueueController.
 class ProtectedMediaIdentifierPermissionContext
     : public PermissionContextBase {
  public:
   explicit ProtectedMediaIdentifierPermissionContext(Profile* profile);
   ~ProtectedMediaIdentifierPermissionContext() override;
 
   // PermissionContextBase implementation.
 #if defined(OS_CHROMEOS)
@@ skipping 42 matching lines @@
 
   // Must be the last member, to ensure that it will be
   // destroyed first, which will invalidate weak pointers
   base::WeakPtrFactory<ProtectedMediaIdentifierPermissionContext> weak_factory_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(ProtectedMediaIdentifierPermissionContext);
 };
 
 #endif  // CHROME_BROWSER_MEDIA_PROTECTED_MEDIA_IDENTIFIER_PERMISSION_CONTEXT_H_