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

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 21d248eb8cde056b016627a4e94106fa690f3d33..f84ee6c214d35b7f9146179db547626cb882b602 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -65,6 +65,7 @@
 #include "ash/wm/window_animations.h"
 #include "ash/wm/window_cycle_controller.h"
 #include "ash/wm/window_properties.h"
+#include "ash/wm/window_selector_controller.h"
 #include "ash/wm/window_util.h"
 #include "ash/wm/workspace_controller.h"
 #include "base/bind.h"
@@ -570,6 +571,7 @@ void Shell::Init() {
   high_contrast_controller_.reset(new HighContrastController);
   video_detector_.reset(new VideoDetector);
   window_cycle_controller_.reset(new WindowCycleController(activation_client_));
+  window_selector_controller_.reset(new WindowSelectorController());
 
   tooltip_controller_.reset(new views::corewm::TooltipController(
                                 gfx::SCREEN_TYPE_ALTERNATE));