Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

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 c0827cc62a552e71d2e7ec8c9a4ba537e2c24abf..0ddceba050b2fa3c06070e2dd52c8ee9eecb3b09 100644
--- a/ash/metrics/task_switch_metrics_recorder.h
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -24,6 +24,8 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
   enum TaskSwitchSource {
     // All task switches caused by shelf buttons, not including sub-menus.
     kShelf,
+    // All task switches caused by the tab strip.
+    kTabStrip,
     // Task switches caused by the WindowCycleController (ie Alt+Tab).
     kWindowCycleController
   };