Add UMA metrics for window selection and overview.

ash/wm/overview/window_overview.h

 // Copyright 2013 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_OVERVIEW_WINDOW_OVERVIEW_H_
 #define ASH_WM_OVERVIEW_WINDOW_OVERVIEW_H_
 
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/scoped_vector.h"
+#include "base/time/time.h"
 #include "ui/events/event_handler.h"
 
 namespace aura {
 class Window;
 class RootWindow;
 }
 
 namespace ui {
 class LocatedEvent;
 }
@@ skipping 63 matching lines @@
   WindowSelectorItemList* windows_;
 
   // Widget indicating which window is currently selected.
   scoped_ptr<views::Widget> selection_widget_;
 
   // If NULL, each root window displays an overview of the windows in that
   // display. Otherwise, all windows are in a single overview on
   // |single_root_window_|.
   aura::RootWindow* single_root_window_;
 
+  // The time when overview was started.
+  base::Time overview_start_time_;
+
   DISALLOW_COPY_AND_ASSIGN(WindowOverview);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_OVERVIEW_WINDOW_OVERVIEW_H_