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

ash/test/task_switch_time_tracker_test_api.cc

Index: ash/test/task_switch_time_tracker_test_api.cc
diff --git a/ash/test/task_switch_time_tracker_test_api.cc b/ash/test/task_switch_time_tracker_test_api.cc
new file mode 100644
index 0000000000000000000000000000000000000000..797207e134574a88bdfb36c8daf9b4fda62c4814
--- /dev/null
+++ b/ash/test/task_switch_time_tracker_test_api.cc
@@ -0,0 +1,33 @@
+// 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.
+
+#include "ash/test/task_switch_time_tracker_test_api.h"
+
+#include "ash/metrics/task_switch_time_tracker.h"
+#include "base/test/simple_test_tick_clock.h"
+
+namespace ash {
+namespace test {
+
+TaskSwitchTimeTrackerTestAPI::TaskSwitchTimeTrackerTestAPI(
+    const std::string& histogram_name) {
+  tick_clock_ = new base::SimpleTestTickClock();
+  time_tracker_.reset(new TaskSwitchTimeTracker(
+      histogram_name, scoped_ptr<base::TickClock>(tick_clock_)));
+}
+
+TaskSwitchTimeTrackerTestAPI::~TaskSwitchTimeTrackerTestAPI() {
+  tick_clock_ = nullptr;
+}
+
+void TaskSwitchTimeTrackerTestAPI::Advance(base::TimeDelta time_delta) {
+  tick_clock_->Advance(time_delta);
+}
+
+bool TaskSwitchTimeTrackerTestAPI::HasLastActionTime() const {
+  return time_tracker_->HasLastActionTime();
+}
+
+}  // namespace test
+}  // namespace ash