[mojo-edk] Ensure there is only one ProcessIncomingMessages() task posted to the IO thread.

mojo/edk/system/node_controller.h

Index: mojo/edk/system/node_controller.h
diff --git a/mojo/edk/system/node_controller.h b/mojo/edk/system/node_controller.h
index c96b4f4405abfbffc79c0f666699b9b9e601c91e..8569834d1851d9d4417a82f306e9dfc3874e8a23 100644
--- a/mojo/edk/system/node_controller.h
+++ b/mojo/edk/system/node_controller.h
@@ -275,9 +275,12 @@ class NodeController : public ports::NodeDelegate,
   std::unordered_map<ports::NodeName, OutgoingMessageQueue>
       pending_relay_messages_;
 
-  // Guards |incoming_messages_|.
+  // Guards |incoming_messages_| and |incoming_messages_task_posted_|.
   base::Lock messages_lock_;
   std::queue<ports::ScopedMessage> incoming_messages_;
+  // Ensures that there is only one incoming messages task posted to the IO
+  // thread.
+  bool incoming_messages_task_posted_ = false;
   // Flag to fast-path checking |incoming_messages_|.
   AtomicFlag incoming_messages_flag_;