Add a mechanism to extend a libevent based message pump.

message_loop/message_loop.h

Index: message_loop/message_loop.h
diff --git a/message_loop/message_loop.h b/message_loop/message_loop.h
index fbb8309c203275768188b499ef5d5e8102a751e9..a146202f0b270ee76bf6a09f3ab53f37de9b85c6 100644
--- a/message_loop/message_loop.h
+++ b/message_loop/message_loop.h
@@ -602,6 +602,12 @@ class BASE_EXPORT MessageLoopForUI : public MessageLoop {
       MessagePumpLibevent::FileDescriptorWatcher* controller,
       MessagePumpLibevent::Watcher* delegate);
 #endif
+
+#if defined(USE_X11)
+  // Please see MessagePumpLibevent for definition.
+  void SetEventSource(MessagePumpLibevent::EventSource* event_source);
+  void ClearEventSource();
+#endif
 };
 
 // Do not add any member variables to MessageLoopForUI!  This is important b/c