Add check to identify the cause of the crash.

ash/common/system/tray/system_tray.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_COMMON_SYSTEM_TRAY_SYSTEM_TRAY_H_
 #define ASH_COMMON_SYSTEM_TRAY_SYSTEM_TRAY_H_
 
 #include <map>
 #include <memory>
 #include <vector>
@@ skipping 265 matching lines @@
   TrayAccessibility* tray_accessibility_;
   TrayAudio* tray_audio_;  // May be null.
   TrayCast* tray_cast_;
   TrayDate* tray_date_;
   TrayUpdate* tray_update_;
 
   // A reference to the Screen share and capture item.
   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_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemTray);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_TRAY_SYSTEM_TRAY_H_