Added metrics to track the time between task switches done via Alt+Tab and Alt+Shift+Tab.

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 28dbd265cd660c5dc30fbfd7d70c8668ce397cd8..806f8d5c71415a6d43648f3e789b4f0e5ff644ba 100644
--- a/ash/metrics/user_metrics_recorder.h
+++ b/ash/metrics/user_metrics_recorder.h
@@ -145,6 +145,10 @@ class ASH_EXPORT UserMetricsRecorder {
   // Records an Ash owned user action.
   void RecordUserMetricsAction(ash::UserMetricsAction action);
 
+  TaskSwitchMetricsRecorder& task_switch_metrics_recorder() {
+    return task_switch_metrics_recorder_;
+  }
+
  private:
   friend class test::UserMetricsRecorderTestAPI;