Add user action for changing the active window in Ash overview mode

ash/metrics/user_metrics_recorder.h

Index: ash/metrics/user_metrics_recorder.h
diff --git a/ash/metrics/user_metrics_recorder.h b/ash/metrics/user_metrics_recorder.h
index d9ccf3fdca1a1950ef0dd5bec49597e03f649c8b..968304bdaa2f6103f7eacdc9d4612c4fa4672d75 100644
--- a/ash/metrics/user_metrics_recorder.h
+++ b/ash/metrics/user_metrics_recorder.h
@@ -120,6 +120,9 @@ enum UserMetricsAction {
   // Thumbnail sized overview of windows triggered by pressing the overview key.
   UMA_WINDOW_OVERVIEW,
 
+  // Change in active window between entering and exiting overview mode.
+  UMA_WINDOW_OVERVIEW_ACTIVE_WINDOW_CHANGED,
+
   // Selecting a window in overview mode by pressing the enter key.
   UMA_WINDOW_OVERVIEW_ENTER_KEY,