Record time between switching the active window from Ash overview mode

ash/metrics/task_switch_metrics_recorder.h

Index: ash/metrics/task_switch_metrics_recorder.h
diff --git a/ash/metrics/task_switch_metrics_recorder.h b/ash/metrics/task_switch_metrics_recorder.h
index 0ddceba050b2fa3c06070e2dd52c8ee9eecb3b09..8d5a881b69893b7512e54b5ad9dda3ee7b058c50 100644
--- a/ash/metrics/task_switch_metrics_recorder.h
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -22,6 +22,9 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
   // Enumeration of the different user interfaces that could be the source of
   // a task switch. Note this is not necessarily comprehensive of all sources.
   enum TaskSwitchSource {
+    // Task switches caused by selecting a window from overview mode which is
+    // different from the previously-active window.
+    kOverviewMode,
     // All task switches caused by shelf buttons, not including sub-menus.
     kShelf,
     // All task switches caused by the tab strip.