Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

ash/metrics/user_metrics_recorder.h

Index: ash/metrics/user_metrics_recorder.h
diff --git a/ash/metrics/user_metrics_recorder.h b/ash/metrics/user_metrics_recorder.h
index 7bb6f9d52d992bd59518523a4cd44733c4f2bb23..66005c3b3d5b300fb3f6d8c893facc8d1c930f1d 100644
--- a/ash/metrics/user_metrics_recorder.h
+++ b/ash/metrics/user_metrics_recorder.h
@@ -7,10 +7,13 @@
 
 #include "ash/ash_export.h"
 #include "ash/metrics/task_switch_metrics_recorder.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/timer/timer.h"
 
 namespace ash {
 
+class UserMetricsRecorderProxy;
+
 namespace test {
 class UserMetricsRecorderTestAPI;
 }
@@ -164,6 +167,8 @@ class ASH_EXPORT UserMetricsRecorder {
   // missing piece is a suitable base::RepeatingTimer test double.
   explicit UserMetricsRecorder(bool record_periodic_metrics);
 
+  void Init(bool record_periodic_metrics);
+
   // Records UMA metrics. Invoked periodically by the |timer_|.
   void RecordPeriodicMetrics();
 
@@ -179,6 +184,8 @@ class ASH_EXPORT UserMetricsRecorder {
 
   TaskSwitchMetricsRecorder task_switch_metrics_recorder_;
 
+  scoped_ptr<UserMetricsRecorderProxy> user_metrics_recorder_proxy_;
+
   DISALLOW_COPY_AND_ASSIGN(UserMetricsRecorder);
 };