Destroy ArcNotificationManager before shutting down MessageCenter.

credit to jamescook@ for the fix

Bug: 875031
Change-Id: I4f1f0c10e45d768da0515400d0666a08937c004b
Reviewed-on: https://chromium-review.googlesource.com/1178935Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584094}

ash/message_center/message_center_controller.cc

@@ -155,10 +155,13 @@ MessageCenterController::MessageCenterController() {
 }
 
 MessageCenterController::~MessageCenterController() {
+  // These members all depend on the MessageCenter instance, so must be
+  // destroyed first.
   all_popup_blocker_.reset();
   session_state_notification_blocker_.reset();
   inactive_user_notification_blocker_.reset();
   fullscreen_notification_blocker_.reset();
+  arc_notification_manager_.reset();
 
   message_center::MessageCenter::Shutdown();
 }