Fix UAF in new Mojo EDK.

mojo/edk/system/broker.h

Index: mojo/edk/system/broker.h
diff --git a/mojo/edk/system/broker.h b/mojo/edk/system/broker.h
index f78fff78ee31ff1d6f80da5b98be043595c0f63b..3b53bae4e82d2126f3d50b376476cb3b5d2ec9d9 100644
--- a/mojo/edk/system/broker.h
+++ b/mojo/edk/system/broker.h
@@ -55,6 +55,8 @@ class MOJO_SYSTEM_IMPL_EXPORT Broker {
 
   // Called by |message_pipe| when it's closing so that its route can be
   // unregistered.
+  // It's ok to call this from a callback (i.e. from OnError or
+  // GotNonTransferableChannel).
   virtual void CloseMessagePipe(uint64_t pipe_id,
                                 MessagePipeDispatcher* message_pipe) = 0;
 };