Make it possible to remove an Awakable with a specific "context" from a Dispatcher.

mojo/edk/system/local_message_pipe_endpoint.h

Index: mojo/edk/system/local_message_pipe_endpoint.h
diff --git a/mojo/edk/system/local_message_pipe_endpoint.h b/mojo/edk/system/local_message_pipe_endpoint.h
index 6649b1cc4f816fb0563abdb62ea18b89b60aab75..7e9904c35a01873384afb9dffa281f95d9432318 100644
--- a/mojo/edk/system/local_message_pipe_endpoint.h
+++ b/mojo/edk/system/local_message_pipe_endpoint.h
@@ -44,6 +44,9 @@ class LocalMessagePipeEndpoint final : public MessagePipeEndpoint {
                          HandleSignalsState* signals_state) override;
   void RemoveAwakable(Awakable* awakable,
                       HandleSignalsState* signals_state) override;
+  void RemoveAwakableWithContext(Awakable* awakable,
+                                 uint64_t context,
+                                 HandleSignalsState* signals_state) override;
 
   // This is only to be used by |MessagePipe|:
   MessageInTransitQueue* message_queue() { return &message_queue_; }