Added metrics to track the time between task switches done via the shelf.

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 d9ccf3fdca1a1950ef0dd5bec49597e03f649c8b..28dbd265cd660c5dc30fbfd7d70c8668ce397cd8 100644
--- a/ash/metrics/user_metrics_recorder.h
+++ b/ash/metrics/user_metrics_recorder.h
@@ -6,6 +6,7 @@
 #define ASH_METRICS_USER_METRICS_RECORDER_H_
 
 #include "ash/ash_export.h"
+#include "ash/metrics/task_switch_metrics_recorder.h"
 #include "base/timer/timer.h"
 
 namespace ash {
@@ -168,6 +169,8 @@ class ASH_EXPORT UserMetricsRecorder {
   // The periodic timer that triggers metrics to be recorded.
   base::RepeatingTimer<UserMetricsRecorder> timer_;
 
+  TaskSwitchMetricsRecorder task_switch_metrics_recorder_;
+
   DISALLOW_COPY_AND_ASSIGN(UserMetricsRecorder);
 };