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

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 b2103be01d7a27a036cec32b1feaf9123f57db25..b37c63b18370b8b1562fe385051ad75d1177ec91 100644
--- a/ash/metrics/task_switch_metrics_recorder.h
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -10,6 +10,10 @@
 #include "ash/ash_export.h"
 #include "base/containers/scoped_ptr_hash_map.h"
 
+namespace aura {
+class Window;
+}  // namespace aura
+
 namespace ash {
 
 class TaskSwitchTimeTracker;
@@ -24,6 +28,9 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
   enum TaskSwitchSource {
     // Task switches from selecting items in the app list.
     kAppList,
+    // Task switches caused by the user clicking or tapping a window in the
+    // 2-dimensional screen space.
+    kDesktop,
     // Task switches caused by selecting a window from overview mode which is
     // different from the previously-active window.
     kOverviewMode,