Add window overview mode behind --ash-enable-overview-mode flag to F5 key.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 69b8970ec19191c23e7858b1637204242c923b79..b3cc1fdd3c469ec7610cdba875c11445e27228c8 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -97,6 +97,7 @@ class UserWallpaperDelegate;
 class VideoDetector;
 class WebNotificationTray;
 class WindowCycleController;
+class WindowSelectorController;
 
 namespace internal {
 class AcceleratorFilter;
@@ -319,6 +320,9 @@ class ASH_EXPORT Shell
   WindowCycleController* window_cycle_controller() {
     return window_cycle_controller_.get();
   }
+  WindowSelectorController* window_selector_controller() {
+    return window_selector_controller_.get();
+  }
   internal::FocusCycler* focus_cycler() {
     return focus_cycler_.get();
   }
@@ -547,6 +551,7 @@ class ASH_EXPORT Shell
   scoped_ptr<UserActivityDetector> user_activity_detector_;
   scoped_ptr<VideoDetector> video_detector_;
   scoped_ptr<WindowCycleController> window_cycle_controller_;
+  scoped_ptr<WindowSelectorController> window_selector_controller_;
   scoped_ptr<internal::FocusCycler> focus_cycler_;
   scoped_ptr<DisplayController> display_controller_;
   scoped_ptr<HighContrastController> high_contrast_controller_;