Added UMA statistics for changing the active window via click or touch events.

ash/metrics/task_switch_metrics_recorder.cc

Index: ash/metrics/task_switch_metrics_recorder.cc
diff --git a/ash/metrics/task_switch_metrics_recorder.cc b/ash/metrics/task_switch_metrics_recorder.cc
index b8b57fe4753606428c68f2be1236c100070d10c9..11ba222f9a724fb758d2d315e9a8d19dbe33c1b8 100644
--- a/ash/metrics/task_switch_metrics_recorder.cc
+++ b/ash/metrics/task_switch_metrics_recorder.cc
@@ -10,6 +10,9 @@ namespace ash {
 
 namespace {
 
+const char kDesktopHistogramName[] =
+    "Ash.Desktop.TimeBetweenNavigateToTaskSwitches";
+
 const char kShelfHistogramName[] =
     "Ash.Shelf.TimeBetweenNavigateToTaskSwitches";
 
@@ -30,6 +33,8 @@ const char* GetHistogramName(
   switch (task_switch_source) {
     case TaskSwitchMetricsRecorder::kAppList:
       return kAppListHistogramName;
+    case TaskSwitchMetricsRecorder::kDesktop:
+      return kDesktopHistogramName;
     case TaskSwitchMetricsRecorder::kOverviewMode:
       return kOverviewModeHistogramName;
     case TaskSwitchMetricsRecorder::kShelf: