Added metrics to track the time between task switches done via Alt+Tab and Alt+Shift+Tab.

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 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 {
     // All task switches caused by shelf buttons, not including sub-menus.
-    kShelf
+    kShelf,
+    // Task switches caused by the WindowCycleController (ie Alt+Tab).
+    kWindowCycleController
   };
 
   TaskSwitchMetricsRecorder();
   virtual ~TaskSwitchMetricsRecorder();
 
   // Notifies |this| that a "navigate to" task switch has occurred. A
   // "navigate to" operation is defined by a task switch where the specific task
   // that becomes active is user-predictable (ie Alt+Tab accelerator, launching
   // a new window via the shelf, etc). Contrast to a "navigate away" operation
   // which is defined as a user interaction that navigates away from a specified
@@ skipping 19 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_