Added Ash.TimeBetweenTaskSwitches to record time deltas between all recorded task switches.

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
index b2103be01d7a27a036cec32b1feaf9123f57db25..aafc9f10258a2aacd72ab59358ffb533315c1057 100644
--- a/ash/metrics/task_switch_metrics_recorder.h
+++ b/ash/metrics/task_switch_metrics_recorder.h
@@ -22,6 +22,9 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
   // 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 {
+    // Task switches between any two other sources. NOTE: This value should NOT

[tdanderson, 2015/06/02 15:56:12]

nit: suggested reword: "Task switches caused by any of the sources in this enum"
?

[bruthig, 2015/06/04 18:54:25]

Done.

+    // be used outside of this class.
+    kAny,
     // Task switches from selecting items in the app list.
     kAppList,
     // Task switches caused by selecting a window from overview mode which is
@@ -48,9 +51,16 @@ class ASH_EXPORT TaskSwitchMetricsRecorder {
   //
   // Will add an entry to |histogram_map_| when called for the first time for
   // each |task_switch_source| value.
+  //
+  // NOTE: A |task_switch_source| value of TaskSwitchSource::kAny should not be
+  // used and behavior is undefined if it is.

[tdanderson, 2015/06/02 15:56:12]

Include in the documentation of OnTaskSwitch() that it does two things: notifies
of a task switch corresponding to the specific source passed in, and also
notifies of an "any" task switch.

[bruthig, 2015/06/04 18:54:26]

Done.

   void OnTaskSwitch(TaskSwitchSource task_switch_source);
 
  private:
+  // Internal Implementation of OnTaskSwitch(TaskSwitchSource) that will accept

[tdanderson, 2015/06/02 15:56:11]

nit: lower-case i on implementation.

[bruthig, 2015/06/04 18:54:26]

Done.

+  // the TaskSwitchSource::kAny value.
+  void OnTaskSwitchInternal(TaskSwitchSource task_switch_source);
+
   // Returns the TaskSwitchTimeTracker associated with the specified
   // |task_switch_source|. May return nullptr if mapping does not exist yet.
   TaskSwitchTimeTracker* FindTaskSwitchTimeTracker(