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

mojo/edk/system/shared_buffer_dispatcher.cc

Index: mojo/edk/system/shared_buffer_dispatcher.cc
diff --git a/mojo/edk/system/shared_buffer_dispatcher.cc b/mojo/edk/system/shared_buffer_dispatcher.cc
index 3ea46d806c61426b4698f314b288945a632e1d3e..ee428cdf728a8107c0530693955f4d2e21a07456 100644
--- a/mojo/edk/system/shared_buffer_dispatcher.cc
+++ b/mojo/edk/system/shared_buffer_dispatcher.cc
@@ -216,8 +216,13 @@ void SharedBufferDispatcher::CloseImplNoLock() {
 }
 
 RefPtr<Dispatcher>
-SharedBufferDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock() {
+SharedBufferDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock(
+    MessagePipe* /*message_pipe*/,
+    unsigned /*port*/) {
   mutex().AssertHeld();
+
+  CancelAllAwakablesNoLock();
+
   DCHECK(shared_buffer_);
   return CreateInternal(std::move(shared_buffer_));
 }