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..c9da4b2bccbd102519136a056e90e694028ef327 100644
--- a/ash/metrics/task_switch_metrics_recorder.cc
+++ b/ash/metrics/task_switch_metrics_recorder.cc
@@ -5,11 +5,16 @@
 #include "ash/metrics/task_switch_metrics_recorder.h"
 
 #include "ash/metrics/task_switch_time_tracker.h"
+#include "ui/aura/window.h"
+#include "ui/wm/public/window_types.h"
 
 namespace ash {
 
 namespace {
 
+const char kDesktopHistogramName[] =
+    "Ash.Desktop.TimeBetweenNavigateToTaskSwitches";
+
 const char kShelfHistogramName[] =
     "Ash.Shelf.TimeBetweenNavigateToTaskSwitches";
 
@@ -30,6 +35,8 @@ 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: