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

mojo/edk/system/message_pipe_endpoint.h

Index: mojo/edk/system/message_pipe_endpoint.h
diff --git a/mojo/edk/system/message_pipe_endpoint.h b/mojo/edk/system/message_pipe_endpoint.h
index f2f38a5afc4c90a8edd5415ad05e57d83401876a..9c1d546e9acff8f2de9edbc5e8b5260661b38e01 100644
--- a/mojo/edk/system/message_pipe_endpoint.h
+++ b/mojo/edk/system/message_pipe_endpoint.h
@@ -72,6 +72,9 @@ class MessagePipeEndpoint {
                                  HandleSignalsState* signals_state);
   virtual void RemoveAwakable(Awakable* awakable,
                               HandleSignalsState* signals_state);
+  virtual void RemoveAwakableWithContext(Awakable* awakable,
+                                         uint64_t context,
+                                         HandleSignalsState* signals_state);
 
   // Implementations must override these if they represent a proxy endpoint. An
   // implementation for a local endpoint needs not override these methods, since