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..78933afe26b03f431365178412dc0260854ac097 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 whwn crbug.com/651242 is fixed.

[afakhry, 2016/10/05 23:32:02]

Nit: whwn --> when

[oshima, 2016/10/06 20:43:53]

Done.

+  bool activating_ = false;
+
   std::unique_ptr<KeyEventWatcher> key_event_watcher_;
 
   std::unique_ptr<ActivationObserver> activation_observer_;