Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

ash/metrics/task_switch_time_tracker.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_TIME_TRACKER_H_
 #define ASH_METRICS_TASK_SWITCH_TIME_TRACKER_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 
 namespace base {
 class HistogramBase;
 class TickClock;
 }
 
 namespace ash {
 
 namespace test {
-class TaskSwitchTimeTrackerTestAPI;
+class ASH_EXPORT TaskSwitchTimeTrackerTestAPI;
 }  // namespace test
 
 // Tracks time deltas between task switches and records them in a histogram.
 class ASH_EXPORT TaskSwitchTimeTracker {
  public:
   // Create a TaskSwitchTimeTracker that will record data to the histogram with
   // the given |histogram_name|.
   explicit TaskSwitchTimeTracker(const std::string& histogram_name);
 
   ~TaskSwitchTimeTracker();
@@ skipping 35 matching lines @@
 
   // The clock used to determine the |last_action_time_|.
   scoped_ptr<base::TickClock> tick_clock_;
 
   DISALLOW_COPY_AND_ASSIGN(TaskSwitchTimeTracker);
 };
 
 }  // namespace ash
 
 #endif  // ASH_METRICS_TASK_SWITCH_TIME_TRACKE_H_