mustash: Fix crash when opening system tray menu

ash/common/system/tray/system_tray.cc

 // 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.
 
 #include "ash/common/system/tray/system_tray.h"
 
 #include "ash/common/key_event_watcher.h"
 #include "ash/common/login_status.h"
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/session/session_state_delegate.h"
@@ skipping 781 matching lines @@
 void SystemTray::ActivateAndStartNavigation(const ui::KeyEvent& key_event) {
   if (!system_bubble_)
     return;
   ActivateBubble();
   views::Widget* widget = GetSystemBubble()->bubble_view()->GetWidget();
   widget->GetFocusManager()->OnKeyEvent(key_event);
 }
 
 void SystemTray::CreateKeyEventWatcher() {
   key_event_watcher_ = WmShell::Get()->CreateKeyEventWatcher();
+  // mustash does not yet support KeyEventWatcher. http://crbug.com/649600.
+  if (!key_event_watcher_)
+    return;
   key_event_watcher_->AddKeyEventCallback(
       ui::Accelerator(ui::VKEY_ESCAPE, ui::EF_NONE),
       base::Bind(&SystemTray::CloseBubble, base::Unretained(this)));
   key_event_watcher_->AddKeyEventCallback(
       ui::Accelerator(ui::VKEY_TAB, ui::EF_NONE),
       base::Bind(&SystemTray::ActivateAndStartNavigation,
                  base::Unretained(this)));
   key_event_watcher_->AddKeyEventCallback(
       ui::Accelerator(ui::VKEY_TAB, ui::EF_SHIFT_DOWN),
       base::Bind(&SystemTray::ActivateAndStartNavigation,
@@ skipping 62 matching lines @@
           .work_area()
           .height();
   if (work_area_height > 0) {
     UMA_HISTOGRAM_CUSTOM_COUNTS(
         "Ash.SystemMenu.PercentageOfWorkAreaHeightCoveredByMenu",
         100 * bubble_view->height() / work_area_height, 1, 300, 100);
   }
 }
 
 }  // namespace ash