Mojo: Only notify delegate if port is still good.

As-is, a late UserMessageReadAckEvent will call
Delegate::PortStatusChanged with an invalid PortRef.

Change-Id: I9f089df40692fc2c39c2de035b6890a1d2995e28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779183
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#692213}

mojo/core/ports/node.cc

@@ -962,7 +962,8 @@ int Node::OnUserMessageReadAck(std::unique_ptr<UserMessageReadAckEvent> event) {
   if (ack_request_event)
     delegate_->ForwardEvent(peer_node_name, std::move(ack_request_event));
 
-  delegate_->PortStatusChanged(port_ref);
+  if (port_ref.is_valid())
+    delegate_->PortStatusChanged(port_ref);
 
   return OK;
 }