Mojo: Don't allow closed pipe handles to be sent

mojo/edk/system/message_pipe_dispatcher.cc

Index: mojo/edk/system/message_pipe_dispatcher.cc
diff --git a/mojo/edk/system/message_pipe_dispatcher.cc b/mojo/edk/system/message_pipe_dispatcher.cc
index d26c912c6e3e59e3975c2a87103f3bcbbecec118..c7b4a32da6bad0e0cbea48ddc8b3e60683a01966 100644
--- a/mojo/edk/system/message_pipe_dispatcher.cc
+++ b/mojo/edk/system/message_pipe_dispatcher.cc
@@ -487,7 +487,7 @@ bool MessagePipeDispatcher::EndSerialize(void* destination,
 
 bool MessagePipeDispatcher::BeginTransit() {
   base::AutoLock lock(signal_lock_);
-  if (in_transit_)
+  if (in_transit_ || port_closed_)
     return false;
   in_transit_ = true;
   return in_transit_;