Revert of Remove IPC::BrokerableAttachment.

ipc/ipc_platform_file_attachment_posix.h

 // Copyright (c) 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 IPC_IPC_PLATFORM_FILE_ATTACHMENT_H_
 #define IPC_IPC_PLATFORM_FILE_ATTACHMENT_H_
 
 #include "ipc/ipc_export.h"
 #include "ipc/ipc_message_attachment.h"
 
 namespace IPC {
 namespace internal {
 
 // A platform file that is sent over |Channel| as a part of |Message|.
 // PlatformFileAttachment optionally owns the file and |owning_| is set in that
 // case. Also, |file_| is not cleared even after the ownership is taken.
 // Some old clients require this strange behavior.
 class IPC_EXPORT PlatformFileAttachment : public MessageAttachment {
  public:
   // Non-owning constructor
   explicit PlatformFileAttachment(base::PlatformFile file);
   // Owning constructor
   explicit PlatformFileAttachment(base::ScopedFD file);
 
   Type GetType() const override;
-  base::PlatformFile TakePlatformFile();
+  base::PlatformFile TakePlatformFile() override;
 
   base::PlatformFile file() const { return file_; }
   bool Owns() const { return owning_.is_valid(); }
 
  private:
   ~PlatformFileAttachment() override;
 
   base::PlatformFile file_;
   base::ScopedFD owning_;
 };
 
 base::PlatformFile GetPlatformFile(scoped_refptr<MessageAttachment> attachment);
 
 }  // namespace internal
 }  // namespace IPC
 
 #endif  // IPC_IPC_PLATFORM_FILE_ATTACHMENT_H_