Fix crash during shutdown

This CL adds protection to ensure a notification can not be
added after ash has been destroyed

Bug: 1164631
Change-Id: I497a142b1f05990a855ce837408adf3660247e19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639161Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845331}

chrome/browser/notifications/chrome_ash_message_center_client.cc

@@ -136,7 +136,13 @@ void ChromeAshMessageCenterClient::Display(
           base::WrapRefCounted(
               new ForwardingNotificationDelegate(notification.id(), delegate_)),
           notification);
-  MessageCenter::Get()->AddNotification(std::move(message_center_notification));
+
+  // During shutdown, Ash is destroyed before |this|, taking the MessageCenter
+  // with it.
+  if (MessageCenter::Get()) {
+    MessageCenter::Get()->AddNotification(
+        std::move(message_center_notification));
+  }
 }
 
 void ChromeAshMessageCenterClient::Close(Profile* profile,