messageTray: Fix warning when opening the tray

As the tray hover handler will call _cancelTrayDwell, we need to
ensure this variable is initialized even if tray dwelling isn't.

https://bugzilla.gnome.org/show_bug.cgi?id=694336

js/ui/messageTray.js

@@ -1754,12 +1754,13 @@ const MessageTray = new Lang.Class({
     },
 
     _setupTrayDwellIfNeeded: function() {
+        this._trayDwellTimeoutId = 0;
+
         // If we don't have extended barrier features, then we need
         // to support the old tray dwelling mechanism.
         if (!global.display.supports_extended_barriers()) {
             let pointerWatcher = PointerWatcher.getPointerWatcher();
             pointerWatcher.addWatch(TRAY_DWELL_CHECK_INTERVAL, Lang.bind(this, this._checkTrayDwell));
-            this._trayDwellTimeoutId = 0;
             this._trayDwelling = false;
             this._trayDwellUserTime = 0;
         }