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

ash/metrics/task_switch_metrics_recorder.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_METRICS_TASK_SWITCH_METRIC_RECORDER_H_
 #define ASH_METRICS_TASK_SWITCH_METRIC_RECORDER_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 #include "base/containers/scoped_ptr_hash_map.h"
 
+namespace aura {
+class Window;
+}  // namespace aura
+
 namespace ash {
 
 class TaskSwitchTimeTracker;
 
 // The TaskSwitchMetricsRecorder class records UMA metrics related to task
 // switching. The main purpose of the TaskSwitchMetricsRecorder is to track time
 // deltas between task switches and record histograms of the deltas.
 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.
   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,

[sky, 2015/06/04 20:10:52]

For the record this style is wrong for chrome code and should be converted.

[bruthig, 2015/06/05 17:28:21]

I've added a TODO.  I will do this in a follow up CL.

     // Task switches caused by selecting a window from overview mode which is
     // different from the previously-active window.
     kOverviewMode,
     // All task switches caused by shelf buttons, not including sub-menus.
     kShelf,
     // All task switches caused by the tab strip.
     kTabStrip,
     // Task switches caused by the WindowCycleController (ie Alt+Tab).
     kWindowCycleController
   };
@@ skipping 28 matching lines @@
   // |histogram_map_| is populated on demand the first time a
   // TaskSwitchTimeTracker is needed for a given source.
   base::ScopedPtrHashMap<int, scoped_ptr<TaskSwitchTimeTracker>> histogram_map_;
 
   DISALLOW_COPY_AND_ASSIGN(TaskSwitchMetricsRecorder);
 };
 
 }  // namespace ash
 
 #endif  // ASH_METRICS_TASK_SWITCH_METRIC_RECORDER_H_