Add UMA metrics for window selection and overview.

ash/wm/overview/window_selector_controller.h

Index: ash/wm/overview/window_selector_controller.h
diff --git a/ash/wm/overview/window_selector_controller.h b/ash/wm/overview/window_selector_controller.h
index 6b168923f7c7c7a1969a261058a5367f2b4af55f..2bd9697a31a4cf81d6b37abd0c93e5743343c2f5 100644
--- a/ash/wm/overview/window_selector_controller.h
+++ b/ash/wm/overview/window_selector_controller.h
@@ -13,6 +13,7 @@
 #include "ash/wm/overview/window_selector_delegate.h"
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/time/time.h"
 #include "ui/aura/window_observer.h"
 
 namespace aura {
@@ -59,7 +60,11 @@ class ASH_EXPORT WindowSelectorController
  private:
   friend class internal::WindowSelectorTest;
 
+  // Dispatched when window selection begins.
+  void OnSelectionStarted();
+
   scoped_ptr<WindowSelector> window_selector_;
+  base::Time last_selection_time_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowSelectorController);
 };