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

mojo/edk/system/dispatcher.cc

Index: mojo/edk/system/dispatcher.cc
diff --git a/mojo/edk/system/dispatcher.cc b/mojo/edk/system/dispatcher.cc
index 386fa0eae9a213d0e4d5d774d99049df8329f6aa..f3ef810d28b5d917dba9163813cc934bcc7734f5 100644
--- a/mojo/edk/system/dispatcher.cc
+++ b/mojo/edk/system/dispatcher.cc
@@ -526,13 +526,14 @@ void Dispatcher::CloseNoLock() {
   CloseImplNoLock();
 }
 
-RefPtr<Dispatcher> Dispatcher::CreateEquivalentDispatcherAndCloseNoLock() {
+RefPtr<Dispatcher> Dispatcher::CreateEquivalentDispatcherAndCloseNoLock(
+    MessagePipe* message_pipe,
+    unsigned port) {
   mutex_.AssertHeld();
   DCHECK(!is_closed_);
 
   is_closed_ = true;
-  CancelAllAwakablesNoLock();
-  return CreateEquivalentDispatcherAndCloseImplNoLock();
+  return CreateEquivalentDispatcherAndCloseImplNoLock(message_pipe, port);
 }
 
 void Dispatcher::StartSerialize(Channel* channel,