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

ipc/ipc_channel_posix.h

Index: ipc/ipc_channel_posix.h
diff --git a/ipc/ipc_channel_posix.h b/ipc/ipc_channel_posix.h
index 8760135209ee7a0a27dbb098a2b6c6bd7cc49a31..be47705db0178f84ccdf7b7e5861c5a813b21c92 100644
--- a/ipc/ipc_channel_posix.h
+++ b/ipc/ipc_channel_posix.h
@@ -29,8 +29,7 @@ class IPC_EXPORT ChannelPosix : public Channel,
   // |broker| must outlive the newly created object.
   ChannelPosix(const IPC::ChannelHandle& channel_handle,
                Mode mode,
-               Listener* listener,
-               AttachmentBroker* broker);
+               Listener* listener);
   ~ChannelPosix() override;
 
   // Channel implementation
@@ -182,9 +181,6 @@ class IPC_EXPORT ChannelPosix : public Channel,
   static int global_pid_;
 #endif  // OS_LINUX
 
-  // |broker_| must outlive this instance.
-  AttachmentBroker* broker_;
-
   DISALLOW_IMPLICIT_CONSTRUCTORS(ChannelPosix);
 };