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

mojo/edk/system/local_message_pipe_endpoint.cc

Index: mojo/edk/system/local_message_pipe_endpoint.cc
diff --git a/mojo/edk/system/local_message_pipe_endpoint.cc b/mojo/edk/system/local_message_pipe_endpoint.cc
index 8e65486e3ba607095fbb1f20c466c491ab83012a..276a009df0cc8490a854a99114d377eec8ad0e5f 100644
--- a/mojo/edk/system/local_message_pipe_endpoint.cc
+++ b/mojo/edk/system/local_message_pipe_endpoint.cc
@@ -183,5 +183,15 @@ void LocalMessagePipeEndpoint::RemoveAwakable(
     *signals_state = GetHandleSignalsState();
 }
 
+void LocalMessagePipeEndpoint::RemoveAwakableWithContext(
+    Awakable* awakable,
+    uint64_t context,
+    HandleSignalsState* signals_state) {
+  DCHECK(is_open_);
+  awakable_list_.RemoveWithContext(awakable, context);
+  if (signals_state)
+    *signals_state = GetHandleSignalsState();
+}
+
 }  // namespace system
 }  // namespace mojo