ChannelMojo: Replace hand written messsages with mojo.

ipc/mojo/client_channel.mojom

Index: ipc/mojo/client_channel.mojom
diff --git a/mojo/shell/external_service.mojom b/ipc/mojo/client_channel.mojom
similarity index 62%
copy from mojo/shell/external_service.mojom
copy to ipc/mojo/client_channel.mojom
index 8cc9e0cae9d2f9a8d66b1716399be1fc39f8fadc..b2da0d40e705f898ca4522339c549c135b997a3e 100644
--- a/mojo/shell/external_service.mojom
+++ b/ipc/mojo/client_channel.mojom
@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module mojo {
+module IPC {
 
-interface ExternalService {
-  Activate(handle<message_pipe>? service_provider_handle);
+interface ClientChannel {
+  Init(handle<message_pipe> pipe, int32 peer_pid) => (int32 pid);
 };
 
 }