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

ash/test/task_switch_time_tracker_test_api.h

Index: ash/test/task_switch_time_tracker_test_api.h
diff --git a/ash/test/task_switch_time_tracker_test_api.h b/ash/test/task_switch_time_tracker_test_api.h
new file mode 100644
index 0000000000000000000000000000000000000000..42144855162ab479e9932106c1201732416b5846
--- /dev/null
+++ b/ash/test/task_switch_time_tracker_test_api.h
@@ -0,0 +1,53 @@
+// 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_TEST_TASK_SWITCH_TIME_TRACKER_TEST_API_H_
+#define ASH_TEST_TASK_SWITCH_TIME_TRACKER_TEST_API_H_
+
+#include <string>
+
+#include "base/memory/scoped_ptr.h"
+#include "base/time/time.h"
+
+namespace base {
+class SimpleTestTickClock;
+}  // namespace base
+
+namespace ash {
+
+class TaskSwitchTimeTracker;
+
+namespace test {
+
+// Provides access TaskSwitchTimeTracker's internals.
+class TaskSwitchTimeTrackerTestAPI {
+ public:
+  // Creates a TaskSwitchTimeTracker with the given |histogram_name| and injects
+  // a base::SimpleTestTickClock that can be controlled.
+  explicit TaskSwitchTimeTrackerTestAPI(const std::string& histogram_name);
+
+  ~TaskSwitchTimeTrackerTestAPI();
+
+  TaskSwitchTimeTracker* time_tracker() { return time_tracker_.get(); }
+
+  // Advances |time_tracker_|'s TickClock by |time_delta|.
+  void Advance(base::TimeDelta time_delta);
+
+  // Wrapper function to access TaskSwitchTimeTracker::HasLastActionTime();
+  bool HasLastActionTime() const;
+
+ private:
+  // Owned by |time_tracker_|.
+  base::SimpleTestTickClock* tick_clock_;
+
+  // The TaskSwitchTimeTracker to provide internal access to.
+  scoped_ptr<TaskSwitchTimeTracker> time_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(TaskSwitchTimeTrackerTestAPI);
+};
+
+}  // namespace test
+}  // namespace ash
+
+#endif  // ASH_TEST_TASK_SWITCH_TIME_TRACKER_TEST_API_H_