Make it possible to write a message pipe endpoint's peer into it.

mojo/edk/system/data_pipe_producer_dispatcher.cc

Index: mojo/edk/system/data_pipe_producer_dispatcher.cc
diff --git a/mojo/edk/system/data_pipe_producer_dispatcher.cc b/mojo/edk/system/data_pipe_producer_dispatcher.cc
index bfa7646026a5047f4f00383c2772483acfa4a4f9..e89f44f26b41da7bf68e34e88cc910418de025cd 100644
--- a/mojo/edk/system/data_pipe_producer_dispatcher.cc
+++ b/mojo/edk/system/data_pipe_producer_dispatcher.cc
@@ -72,9 +72,13 @@ void DataPipeProducerDispatcher::CloseImplNoLock() {
 }
 
 RefPtr<Dispatcher>
-DataPipeProducerDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock() {
+DataPipeProducerDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock(
+    MessagePipe* /*message_pipe*/,
+    unsigned /*port*/) {
   mutex().AssertHeld();
 
+  CancelAllAwakablesNoLock();
+
   auto dispatcher = DataPipeProducerDispatcher::Create();
   dispatcher->Init(std::move(data_pipe_));
   return dispatcher;