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

ash/metrics/task_switch_metrics_recorder.cc

+// 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/metrics/task_switch_metrics_recorder.h"
+
+#include "ash/metrics/task_switch_time_tracker.h"
+
+namespace ash {
+
+namespace {
+
+const char kShelf_histogram[] = "Ash.Shelf.TimeBetweenNavigateToTaskSwitches";
+
+// Returns the histogram name for the given |task_switch_source|.
+const char* GetHistogramName(
+    TaskSwitchMetricsRecorder::TaskSwitchSource task_switch_source) {
+  switch (task_switch_source) {
+    case TaskSwitchMetricsRecorder::kShelf:
+      return kShelf_histogram;
+  }
+  NOTREACHED();
+  return nullptr;
+}
+
+}  // namespace
+
+TaskSwitchMetricsRecorder::TaskSwitchMetricsRecorder() {
+}
+
+TaskSwitchMetricsRecorder::~TaskSwitchMetricsRecorder() {
+}
+
+void TaskSwitchMetricsRecorder::OnTaskSwitch(
+    TaskSwitchSource task_switch_source) {
+  TaskSwitchTimeTracker* task_switch_time_tracker =
+      FindTaskSwitchTimeTracker(task_switch_source);
+  if (!task_switch_time_tracker)
+    AddTaskSwitchTimeTracker(task_switch_source);
+
+  task_switch_time_tracker = FindTaskSwitchTimeTracker(task_switch_source);
+  CHECK(task_switch_time_tracker);
+
+  task_switch_time_tracker->OnTaskSwitch();
+}
+
+TaskSwitchTimeTracker* TaskSwitchMetricsRecorder::FindTaskSwitchTimeTracker(
+    TaskSwitchSource task_switch_source) {
+  return histogram_map_.get(task_switch_source);
+}
+
+void TaskSwitchMetricsRecorder::AddTaskSwitchTimeTracker(
+    TaskSwitchSource task_switch_source) {
+  CHECK(histogram_map_.find(task_switch_source) == histogram_map_.end());
+
+  const char* histogram_name = GetHistogramName(task_switch_source);
+  CHECK(histogram_name);

[oshima, 2015/05/13 23:46:50]

nit: DCHECK will be sufficient unless you want to investigate crashes, or it has
security risk. 

[bruthig, 2015/05/14 14:19:46]

Done.

+
+  histogram_map_.add(task_switch_source,
+                     scoped_ptr<TaskSwitchTimeTracker>(
+                         new TaskSwitchTimeTracker(histogram_name)));
+}
+
+}  // namespace ash