Fix race condition with multiplexed message pipes.

mojo/edk/system/child_broker.h

Index: mojo/edk/system/child_broker.h
diff --git a/mojo/edk/system/child_broker.h b/mojo/edk/system/child_broker.h
index 38cef560c204c31155406c1b1bf6efe4d3634e7e..c2d09b3036e5b5f10a00d539db7b2a275d96d195 100644
--- a/mojo/edk/system/child_broker.h
+++ b/mojo/edk/system/child_broker.h
@@ -70,6 +70,11 @@ class MOJO_SYSTEM_IMPL_EXPORT ChildBroker
   // Initializes |parent_async_channel_|.
   void InitAsyncChannel(ScopedPlatformHandle parent_async_channel_handle);
 
+  // Helper method to connect the given MessagePipe to the channel.
+  void AttachMessagePipe(MessagePipeDispatcher* message_pipe,
+                         uint64_t pipe_id,
+                         RoutedRawChannel* raw_channel);
+
 #if defined(OS_WIN)
   // Helper method to write the given message and read back the result.
   bool WriteAndReadResponse(BrokerMessage* message,