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 87a776007fb7169a12c9361f0c6e0d848665139e..085708b49ac6698035e15ebfd4aead3253161040 100644
--- a/ash/metrics/task_switch_metrics_recorder.h
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -23,7 +23,10 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
   // a task switch. Note this is not necessarily comprehensive of all sources.
   enum TaskSwitchSource {
     // All task switches caused by shelf buttons, not including sub-menus.
-    kShelf
+    kShelf,
+    // Task switches caused by selecting a window from overview mode which is
+    // different from the previously-active window.
+    kOverviewMode
   };
 
   TaskSwitchMetricsRecorder();