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

mojo/edk/system/simple_dispatcher.cc

Index: mojo/edk/system/simple_dispatcher.cc
diff --git a/mojo/edk/system/simple_dispatcher.cc b/mojo/edk/system/simple_dispatcher.cc
index 48ee27d5cb888ca7cd8bbba94e74e0309a491d89..c3dcd61ebce923009cbaceb2ce1af6449f13ce4a 100644
--- a/mojo/edk/system/simple_dispatcher.cc
+++ b/mojo/edk/system/simple_dispatcher.cc
@@ -60,5 +60,15 @@ void SimpleDispatcher::RemoveAwakableImplNoLock(
     *signals_state = GetHandleSignalsStateImplNoLock();
 }
 
+void SimpleDispatcher::RemoveAwakableWithContextImplNoLock(
+    Awakable* awakable,
+    uint64_t context,
+    HandleSignalsState* signals_state) {
+  mutex().AssertHeld();
+  awakable_list_.RemoveWithContext(awakable, context);
+  if (signals_state)
+    *signals_state = GetHandleSignalsStateImplNoLock();
+}
+
 }  // namespace system
 }  // namespace mojo