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

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..c0827cc62a552e71d2e7ec8c9a4ba537e2c24abf 100644
--- a/ash/metrics/task_switch_metrics_recorder.h
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -23,7 +23,9 @@ 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 the WindowCycleController (ie Alt+Tab).
+    kWindowCycleController
   };
 
   TaskSwitchMetricsRecorder();