Added metrics to track the time between task switches done via the shelf.

ash/metrics/task_switch_time_tracker.h

Index: ash/metrics/task_switch_time_tracker.h
diff --git a/ash/metrics/task_switch_time_tracker.h b/ash/metrics/task_switch_time_tracker.h
new file mode 100644
index 0000000000000000000000000000000000000000..77d1182033a60b9b9538bfab2b5d0f8d2401ff0b
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+++ b/ash/metrics/task_switch_time_tracker.h
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+// 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;
+}  // 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();
+
+  // Notifies |this| that a task switch has occurred. A histogram data point
+  // will be recorded for all calls but the first.
+  void OnTaskSwitch();
+
+ private:
+  friend class test::TaskSwitchTimeTrackerTestAPI;
+
+  // Private constructor that the test::TaskSwitchTimeTrackerTestAPI can use to
+  // inject a custom |tick_clock|.
+  TaskSwitchTimeTracker(const std::string& histogram_name,
+                        scoped_ptr<base::TickClock> tick_clock);
+
+  // Returns true if |last_action_time_| has a valid value.
+  bool HasLastActionTime() const;
+
+  // Sets the |last_action_time_| to |tick_clock_|'s current value and returns
+  // the previous value for |last_action_time_|.
+  base::TimeTicks SetLastActionTime();
+
+  // Records a data point in the histogram.
+  void RecordTimeDelta();
+
+  // Lazily obtains and sets the |histogram_|.
+  base::HistogramBase* GetHistogram();
+
+  // The histogram name to record data to.
+  std::string histogram_name_;
+
+  // The histogram to log data to. Set via GetHistogram() using lazy load.
+  base::HistogramBase* histogram_ = nullptr;
+
+  // Tracks the last time OnTaskSwitch() was called. A value of
+  // base::TimeTicks() should be interpreted as not set.
+  base::TimeTicks last_action_time_ = base::TimeTicks();
+
+  // 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_