Add check to identify the cause of the crash.

ash/common/system/tray/system_tray.h

Index: ash/common/system/tray/system_tray.h
diff --git a/ash/common/system/tray/system_tray.h b/ash/common/system/tray/system_tray.h
index 5af32ff7f2c932c875db8b4dece5f1456932b335..92c5da81ba47b4be7cdf6c3d47342d602b7578a1 100644
--- a/ash/common/system/tray/system_tray.h
+++ b/ash/common/system/tray/system_tray.h
@@ -283,6 +283,9 @@ class ASH_EXPORT SystemTray : public TrayBackgroundView,
   ScreenTrayItem* screen_capture_tray_item_;  // not owned
   ScreenTrayItem* screen_share_tray_item_;    // not owned
 
+  // TODO(oshima): Remove this when crbug.com/651242 is fixed.
+  bool activating_ = false;
+
   std::unique_ptr<KeyEventWatcher> key_event_watcher_;
 
   std::unique_ptr<ActivationObserver> activation_observer_;