Mojo: Add Channel::AttachEndpoint(), which takes a ChannelEndpoint.

mojo/system/channel_endpoint.h

Index: mojo/system/channel_endpoint.h
diff --git a/mojo/system/channel_endpoint.h b/mojo/system/channel_endpoint.h
index 5dd34d4aa4009bd4a2a13592bc92840a5e703683..593ab5597f11bb8d00d9a13446b29d8150ca33ac 100644
--- a/mojo/system/channel_endpoint.h
+++ b/mojo/system/channel_endpoint.h
@@ -156,6 +156,10 @@ class MOJO_SYSTEM_IMPL_EXPORT ChannelEndpoint
   friend class base::RefCountedThreadSafe<ChannelEndpoint>;
   ~ChannelEndpoint();
 
+  // Must be called with |lock_| held.
+  bool WriteMessageNoLock(scoped_ptr<MessageInTransit> message);
+
+  // TODO(vtl): Move these under lock.
   State state_;
   // TODO(vtl): When moved under lock, this can/should be made a raw pointer.
   scoped_refptr<MessagePipe> message_pipe_;