Add UMA metrics for window selection and overview.

ash/wm/overview/window_overview.h

Index: ash/wm/overview/window_overview.h
diff --git a/ash/wm/overview/window_overview.h b/ash/wm/overview/window_overview.h
index b8c81dbf72156619bbe6e653dddacbbffd028953..fab13f050dc861c895f057204df8712bacc927cf 100644
--- a/ash/wm/overview/window_overview.h
+++ b/ash/wm/overview/window_overview.h
@@ -8,6 +8,7 @@
 #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 {
@@ -91,6 +92,9 @@ class WindowOverview : public ui::EventHandler {
   // |single_root_window_|.
   aura::RootWindow* single_root_window_;
 
+  // The time when overview was started.
+  base::Time overview_start_time_;
+
   DISALLOW_COPY_AND_ASSIGN(WindowOverview);
 };