ash: make ArcNotificationManagerTest delete objects in TearDown()

To do otherwise means some state lasts longer than the objects created by
Ash. This causes problems with single-process-ash.

BUG=874090
TEST=test only changes

Change-Id: I359dbd5b3be90c1733bc9c94c6ea1c57f4e39513
Reviewed-on: https://chromium-review.googlesource.com/1196921Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587650}

ash/system/message_center/arc/arc_notification_view_unittest.cc

@@ -108,6 +108,9 @@ class ArcNotificationViewTest : public AshTestBase {
   void TearDown() override {
     widget()->Close();
     notification_view_.reset();
+    item_.reset();
+    notification_.reset();
+    surface_.reset();
     AshTestBase::TearDown();
   }
 

testing/buildbot/filters/chromeos.single_process_mash.fyi.ash_unittests.filter

 # These tests fail.
 -AppListPresenterDelegateTest.TapAppListWithVirtualKeyboardDismissesVirtualKeyboard/0
 -AppListPresenterDelegateTest.TapAppListWithVirtualKeyboardDismissesVirtualKeyboard/1
--ArcNotificationViewTest.ChangeContentHeight
--ArcNotificationViewTest.Events
--ArcNotificationViewTest.PressBackspaceKey
--ArcNotificationViewTest.PressBackspaceKeyOnEditBox
--ArcNotificationViewTest.SlideOut
--ArcNotificationViewTest.SlideOutNested
--ArcNotificationViewTest.SlideOutPinned
--ArcNotificationViewTest.SnoozeButton
 -LockActionHandlerLayoutManagerTest.KeyboardBounds
 -ShelfWindowWatcherTest.CreateAndRemoveShelfItemProperties
 -ShellTest.KeyboardCreation