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

ash/metrics/task_switch_metrics_recorder.h

Index: ash/metrics/task_switch_metrics_recorder.h
diff --git a/ash/metrics/task_switch_metrics_recorder.h b/ash/metrics/task_switch_metrics_recorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..87a776007fb7169a12c9361f0c6e0d848665139e
--- /dev/null
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -0,0 +1,65 @@
+// 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_METRIC_RECORDER_H_
+#define ASH_METRICS_TASK_SWITCH_METRIC_RECORDER_H_
+
+#include <string>
+
+#include "ash/ash_export.h"
+#include "base/containers/scoped_ptr_hash_map.h"
+
+namespace ash {
+
+class TaskSwitchTimeTracker;
+
+// The TaskSwitchMetricsRecorder class records UMA metrics related to task
+// switching. The main purpose of the TaskSwitchMetricsRecorder is to track time
+// deltas between task switches and record histograms of the deltas.
+class ASH_EXPORT TaskSwitchMetricsRecorder {
+ public:
+  // Enumeration of the different user interfaces that could be the source of
+  // a task switch. Note this is not necessarily comprehensive of all sources.
+  enum TaskSwitchSource {
+    // All task switches caused by shelf buttons, not including sub-menus.
+    kShelf
+  };
+
+  TaskSwitchMetricsRecorder();
+  virtual ~TaskSwitchMetricsRecorder();
+
+  // Notifies |this| that a "navigate to" task switch has occurred. A
+  // "navigate to" operation is defined by a task switch where the specific task
+  // that becomes active is user-predictable (ie Alt+Tab accelerator, launching
+  // a new window via the shelf, etc). Contrast to a "navigate away" operation
+  // which is defined as a user interaction that navigates away from a specified
+  // task and the next task that becomes active is likely not user-predictable
+  // (ie. closing or minimizing a window, closing a tab, etc).
+  //
+  // Will add an entry to |histogram_map_| when called for the first time for
+  // each |task_switch_source| value.
+  void OnTaskSwitch(TaskSwitchSource task_switch_source);
+
+ private:
+  // Returns the TaskSwitchTimeTracker associated with the specified
+  // |task_switch_source|. May return nullptr if mapping does not exist yet.
+  TaskSwitchTimeTracker* FindTaskSwitchTimeTracker(
+      TaskSwitchSource task_switch_source);
+
+  // Adds a TaskSwitchTimeTracker to |histogram_map_| for the specified
+  // |task_switch_source|. Behavior is undefined if a TaskSwitchTimeTracker
+  // |histogram_map_| already has an entry for |task_switch_source|.
+  void AddTaskSwitchTimeTracker(TaskSwitchSource task_switch_source);
+
+  // Tracks TaskSwitchSource to TaskSwitchTimeTracker mappings. The
+  // |histogram_map_| is populated on demand the first time a
+  // TaskSwitchTimeTracker is needed for a given source.
+  base::ScopedPtrHashMap<int, scoped_ptr<TaskSwitchTimeTracker>> histogram_map_;
+
+  DISALLOW_COPY_AND_ASSIGN(TaskSwitchMetricsRecorder);
+};
+
+}  // namespace ash
+
+#endif  // ASH_METRICS_TASK_SWITCH_METRIC_RECORDER_H_