Fix UAF in new Mojo EDK.

mojo/edk/system/broker.h

 // Copyright 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 MOJO_EDK_SYSTEM_BROKER_H_
 #define MOJO_EDK_SYSTEM_BROKER_H_
 
 #include <stdint.h>
 
 #include "mojo/edk/embedder/scoped_platform_handle.h"
@@ skipping 37 matching lines @@
 
   // Called by |message_pipe| so that it receives messages for the given
   // globally unique |pipe_id|. When the connection is established,
   // MessagePipeDispatcher::GotNonTransferableChannel is called with the channel
   // that it can use for sending messages.
   virtual void ConnectMessagePipe(uint64_t pipe_id,
                                   MessagePipeDispatcher* message_pipe) = 0;
 
   // Called by |message_pipe| when it's closing so that its route can be
   // unregistered.
+  // It's ok to call this from a callback (i.e. from OnError or
+  // GotNonTransferableChannel).
   virtual void CloseMessagePipe(uint64_t pipe_id,
                                 MessagePipeDispatcher* message_pipe) = 0;
 };
 
 }  // namespace edk
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_BROKER_H_