Adds WmShell::RecordTaskSwitchMetric()

ash/common/metrics/task_switch_source.h

Index: ash/common/metrics/task_switch_source.h
diff --git a/ash/common/metrics/task_switch_source.h b/ash/common/metrics/task_switch_source.h
new file mode 100644
index 0000000000000000000000000000000000000000..7f882e54cd3b5c4753f4c4baca4aeb4b87fab25c
--- /dev/null
+++ b/ash/common/metrics/task_switch_source.h
@@ -0,0 +1,34 @@
+// Copyright 2016 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_COMMON_METRICS_TASK_SWITCH_SOURCE_H_
+#define ASH_COMMON_METRICS_TASK_SWITCH_SOURCE_H_
+
+namespace ash {
+
+// 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 class TaskSwitchSource {
+  // Task switches caused by any two sources in this enum. NOTE: This value
+  // should NOT be used outside of this class.

[James Cook, 2016/07/18 22:27:10]

nit: "this class" -> which? or in general, where is it OK to use?

[sky, 2016/07/18 22:43:45]

Ya, sorry, I moved the code without carefully reading the comments. Updated.

+  ANY,
+  // Task switches from selecting items in the app list.
+  APP_LIST,
+  // Task switches caused by the user activating a task window by clicking or
+  // tapping on it.
+  DESKTOP,
+  // Task switches caused by selecting a window from overview mode which is
+  // different from the previously-active window.
+  OVERVIEW_MODE,
+  // All task switches caused by shelf buttons, not including sub-menus.
+  SHELF,
+  // All task switches caused by the tab strip.
+  TAB_STRIP,
+  // Task switches caused by the WindowCycleController (ie Alt+Tab).
+  WINDOW_CYCLE_CONTROLLER
+};
+
+}  // namespace ash
+
+#endif  // ASH_COMMON_METRICS_TASK_SWITCH_SOURCE_H_