Mojo: Add a "GetType()" to Dispatchers.

mojo/system/message_pipe_dispatcher.cc

Index: mojo/system/message_pipe_dispatcher.cc
diff --git a/mojo/system/message_pipe_dispatcher.cc b/mojo/system/message_pipe_dispatcher.cc
index b90bc74f94bb9ce5c43fea67cac48982308246b5..b282ed744ce0eb0b82f5d60e8405ab9484200a73 100644
--- a/mojo/system/message_pipe_dispatcher.cc
+++ b/mojo/system/message_pipe_dispatcher.cc
@@ -27,6 +27,10 @@ void MessagePipeDispatcher::Init(scoped_refptr<MessagePipe> message_pipe,
   port_ = port;
 }
 
+Dispatcher::Type MessagePipeDispatcher::GetType() {
+  return kTypeMessagePipe;
+}
+
 MessagePipeDispatcher::~MessagePipeDispatcher() {
   // |Close()|/|CloseImplNoLock()| should have taken care of the pipe.
   DCHECK(!message_pipe_.get());