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

ash/metrics/user_metrics_recorder.cc

Index: ash/metrics/user_metrics_recorder.cc
diff --git a/ash/metrics/user_metrics_recorder.cc b/ash/metrics/user_metrics_recorder.cc
index dee488bae57d74d65d47624096442116ed06b2ed..3cdbbb669e47f066177e8c1a8de89b7837f69d47 100644
--- a/ash/metrics/user_metrics_recorder.cc
+++ b/ash/metrics/user_metrics_recorder.cc
@@ -231,12 +231,16 @@ void UserMetricsRecorder::RecordUserMetricsAction(UserMetricsAction action) {
       break;
     case ash::UMA_LAUNCHER_LAUNCH_TASK:
       base::RecordAction(base::UserMetricsAction("Launcher_LaunchTask"));
+      task_switch_metrics_recorder_.OnTaskSwitch(
+          TaskSwitchMetricsRecorder::kShelf);
       break;
     case ash::UMA_LAUNCHER_MINIMIZE_TASK:
       base::RecordAction(base::UserMetricsAction("Launcher_MinimizeTask"));
       break;
     case ash::UMA_LAUNCHER_SWITCH_TASK:
       base::RecordAction(base::UserMetricsAction("Launcher_SwitchTask"));
+      task_switch_metrics_recorder_.OnTaskSwitch(
+          TaskSwitchMetricsRecorder::kShelf);
       break;
     case UMA_MAXIMIZE_MODE_DISABLED:
       base::RecordAction(base::UserMetricsAction("Touchview_Disabled"));