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

mojo/edk/system/simple_dispatcher.h

Index: mojo/edk/system/simple_dispatcher.h
diff --git a/mojo/edk/system/simple_dispatcher.h b/mojo/edk/system/simple_dispatcher.h
index 558d42cafa8f29bdbd3e17b01ec73d7849641023..4501e8ca98b61aa053346b95dfaa07e584a3cf02 100644
--- a/mojo/edk/system/simple_dispatcher.h
+++ b/mojo/edk/system/simple_dispatcher.h
@@ -37,6 +37,10 @@ class SimpleDispatcher : public Dispatcher {
                                    HandleSignalsState* signals_state) override;
   void RemoveAwakableImplNoLock(Awakable* awakable,
                                 HandleSignalsState* signals_state) override;
+  void RemoveAwakableWithContextImplNoLock(
+      Awakable* awakable,
+      uint64_t context,
+      HandleSignalsState* signals_state) override;
 
  private:
   AwakableList awakable_list_ MOJO_GUARDED_BY(mutex());