Relanding "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..177ff0807c260b2cce2b3a90ca1229a5d35a2b1e 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;
@@ -21,9 +25,13 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
  public:
   // Enumeration of the different user interfaces that could be the source of
   // a task switch. Note this is not necessarily comprehensive of all sources.
+  // TODO(bruthig): Convert enum format from kValue to VALUE.
   enum TaskSwitchSource {
     // Task switches from selecting items in the app list.
     kAppList,
+    // Task switches caused by the user activating a task window by clicking or
+    // tapping on it.
+    kDesktop,
     // Task switches caused by selecting a window from overview mode which is
     // different from the previously-active window.
     kOverviewMode,