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 579f7f7b2adbf428e6b86da04367a2cdc26b67b0..35a158e3c6da85c13d4cbe6957d8a0a71e832bb7 100644
--- a/ipc/ipc_channel_win.h
+++ b/ipc/ipc_channel_win.h
@@ -32,8 +32,7 @@ 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
@@ -133,9 +132,6 @@ 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);
 };