Fix Chrome crashes on calling OnTrayVisibilityChange

ash/common/system/tray/system_tray_unittest.cc

Index: ash/common/system/tray/system_tray_unittest.cc
diff --git a/ash/common/system/tray/system_tray_unittest.cc b/ash/common/system/tray/system_tray_unittest.cc
index 6a9c270fc2058a587173a9288d6ebcbb172f8e0a..abdd347651cce32ff0aead7bcda7f804493bf01c 100644
--- a/ash/common/system/tray/system_tray_unittest.cc
+++ b/ash/common/system/tray/system_tray_unittest.cc
@@ -60,7 +60,18 @@ class ModalWidgetDelegate : public views::WidgetDelegateView {
 
 }  // namespace
 
-typedef AshTestBase SystemTrayTest;
+class SystemTrayTest : public AshTestBase {
+ public:
+  SystemTrayTest() {}
+  ~SystemTrayTest() override {}
+
+  void ShutDownLogoutButtonTray(StatusAreaWidget* widget) {

[varkha, 2016/12/13 17:34:54]

nit: I think we usually prefer Shutdown over ShutDown.

+    widget->logout_button_tray_ = nullptr;
+  }
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(SystemTrayTest);
+};
 
 // Verifies only the visible default views are recorded in the
 // "Ash.SystemMenu.DefaultView.VisibleItems" histogram.
@@ -664,6 +675,15 @@ TEST_F(SystemTrayTest, SystemTrayHeightWithBubble) {
 
   EXPECT_EQ(0, notification_tray->tray_bubble_height_for_test());
 }
+
+// Calling OnVisibilityChange on StatusAreaWidget that has been Shutdown, see
+// crbug.com/671293. Test if crash is mitigated.
+TEST_F(SystemTrayTest, SystemTrayInitialization) {
+  StatusAreaWidget* widget = StatusAreaWidgetTestHelper::GetStatusAreaWidget();
+  ShutDownLogoutButtonTray(widget);

[varkha, 2016/12/13 17:34:54]

I am still confused if the bad scenario is a result of a call to visibility
change callback while we are inside StatusAreaWidget::CreateTrayViews() but
haven't initialized the |logout_button_tray_| yet (in which case you are rightly
testing it with resetting a single data member) or alternatively if the scenario
is a result of a race between StatusAreaWidget::CreateTrayViews() and
StatusAreaWidget::IsNextVisibleTrayToLogout() where IsNextVisibleTrayToLogout()
can be called before CreateTrayViews() (in which case I would prefer the test to
just invoke StatusAreaWidget::Shutdown() instead to match the real life
scenario.

+  WebNotificationTray* notification_tray = widget->web_notification_tray();
+  widget->OnTrayVisibilityChanged(notification_tray);
+}
 #endif  // OS_CHROMEOS
 
 }  // namespace test