ipc: Use a global for the process's attachment broker.

ipc/ipc_channel_nacl.h

 // Copyright (c) 2012 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_CHANNEL_NACL_H_
 #define IPC_IPC_CHANNEL_NACL_H_
 
 #include <deque>
 #include <string>
 
@@ skipping 102 matching lines @@
   // don't change to something like std::deque<> without changing the
   // implementation!
   std::vector<int> input_fds_;
 
   // This queue is used when a message is sent prior to Connect having been
   // called. Normally after we're connected, the queue is empty.
   std::deque<linked_ptr<Message> > output_queue_;
 
   base::WeakPtrFactory<ChannelNacl> weak_ptr_factory_;
 
-  // |broker_| must outlive this instance.
-  AttachmentBroker* broker_;
-
   DISALLOW_IMPLICIT_CONSTRUCTORS(ChannelNacl);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_CHANNEL_NACL_H_