Use base::MessageLoop in more files.

ash/system/web_notification/web_notification_tray.cc

Index: ash/system/web_notification/web_notification_tray.cc
diff --git a/ash/system/web_notification/web_notification_tray.cc b/ash/system/web_notification/web_notification_tray.cc
index 9c108220bdd77526f54a31d6bdf666ad61cc27ee..4565ef08b0f32f43a789aad4ea4a9ec699c1d38f 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -464,7 +464,7 @@ void WebNotificationTray::OnMessageCenterTrayChanged() {
   // consecutively, and calling Update in the middle of those events will show
   // intermediate unread counts for a moment.
   should_update_tray_content_ = true;
-  MessageLoop::current()->PostTask(
+  base::MessageLoop::current()->PostTask(
       FROM_HERE,
       base::Bind(&WebNotificationTray::UpdateTrayContent, AsWeakPtr()));
 }