CrOS: Do not allow notifications to be added during shutdown.

chrome/browser/background/background_contents_service_unittest.cc

Index: chrome/browser/background/background_contents_service_unittest.cc
diff --git a/chrome/browser/background/background_contents_service_unittest.cc b/chrome/browser/background/background_contents_service_unittest.cc
index 0c3db32ebb0d03ccc0542875898a937ae0671fb8..9efac11a702b283a2d016994aadf9c5ec82af599 100644
--- a/chrome/browser/background/background_contents_service_unittest.cc
+++ b/chrome/browser/background/background_contents_service_unittest.cc
@@ -193,7 +193,7 @@ class BackgroundContentsServiceNotificationTest
   }
 
   void TearDown() override {
-    g_browser_process->notification_ui_manager()->CancelAll();
+    g_browser_process->notification_ui_manager()->StartShutdown();
     profile_manager_.reset();
 #if !defined(OS_CHROMEOS)
     message_center::MessageCenter::Shutdown();