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

ipc/ipc_channel_win.h

Index: ipc/ipc_channel_win.h
diff --git a/ipc/ipc_channel_win.h b/ipc/ipc_channel_win.h
index 9d561b010a56ee77d71057f40213730691a2948e..6fe0a766bc95f687ca55a38b157e35e1449d37ef 100644
--- a/ipc/ipc_channel_win.h
+++ b/ipc/ipc_channel_win.h
@@ -30,8 +30,7 @@ class ChannelWin : public Channel,
   // |broker| must outlive the newly created object.
   ChannelWin(const IPC::ChannelHandle& channel_handle,
              Mode mode,
-             Listener* listener,
-             AttachmentBroker* broker);
+             Listener* listener);
   ~ChannelWin() override;
 
   // Channel implementation
@@ -131,9 +130,6 @@ class ChannelWin : public Channel,
 
   scoped_ptr<base::ThreadChecker> thread_check_;
 
-  // |broker_| must outlive this instance.
-  AttachmentBroker* broker_;
-
   base::WeakPtrFactory<ChannelWin> weak_factory_;
   DISALLOW_COPY_AND_ASSIGN(ChannelWin);
 };