Take a screenshot in maximize mode when volume down and power are pressed.

ash/wm/maximize_mode/maximize_mode_event_blocker.h

 // Copyright 2014 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_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_EVENT_BLOCKER_H_
 #define ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_EVENT_BLOCKER_H_
 
 #include "ash/shell_observer.h"
 #include "base/macros.h"
 #include "base/memory/scoped_vector.h"
 #include "ui/aura/scoped_window_targeter.h"
 
 namespace aura {
 class Window;
 }  // namespace aura
 
 namespace ash {
 
 // A class which blocks mouse and keyboard events while instantiated by
-// replacing the root window event targeter.
+// replacing the root window event targeter. The replacement targeter also
+// handles a volume down + power keypress hotkey to take screenshots as the
+// keyboard cannot be used to do so.
 class MaximizeModeEventBlocker : public ShellObserver {
  public:
   MaximizeModeEventBlocker();
   virtual ~MaximizeModeEventBlocker();
 
   // ShellObserver:
   virtual void OnRootWindowAdded(aura::Window* root_window) OVERRIDE;
 
  private:
   // Adds an event targeter on |root_window| to block mouse and keyboard events.
   void AddEventTargeterOn(aura::Window* root_window);
 
   ScopedVector<aura::ScopedWindowTargeter> targeters_;
 
   DISALLOW_COPY_AND_ASSIGN(MaximizeModeEventBlocker);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_EVENT_BLOCKER_H_