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

ash/metrics/user_metrics_recorder.cc

Index: ash/metrics/user_metrics_recorder.cc
diff --git a/ash/metrics/user_metrics_recorder.cc b/ash/metrics/user_metrics_recorder.cc
index e9929402dd3295ab22e4c73ba238ec256777235c..c3d1ec511d62ce81336d900e3127b1f13fa8a23a 100644
--- a/ash/metrics/user_metrics_recorder.cc
+++ b/ash/metrics/user_metrics_recorder.cc
@@ -4,6 +4,7 @@
 
 #include "ash/metrics/user_metrics_recorder.h"
 
+#include "ash/metrics/user_metrics_recorder_proxy.h"
 #include "ash/session/session_state_delegate.h"
 #include "ash/shelf/shelf_delegate.h"
 #include "ash/shelf/shelf_item_types.h"
@@ -12,6 +13,7 @@
 #include "ash/shelf/shelf_view.h"
 #include "ash/shelf/shelf_widget.h"
 #include "ash/shell.h"
+#include "ash/shell_delegate.h"
 #include "ash/shell_window_ids.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/wm/window_state.h"
@@ -182,10 +184,16 @@ void RecordShelfItemCounts() {
 }  // namespace
 
 UserMetricsRecorder::UserMetricsRecorder() {
-  StartTimer();
+  Init(true);
 }
 
 UserMetricsRecorder::UserMetricsRecorder(bool record_periodic_metrics) {
+  Init(record_periodic_metrics);
+}
+
+void UserMetricsRecorder::Init(bool record_periodic_metrics) {
+  user_metrics_recorder_proxy_.reset(
+      ash::Shell::GetInstance()->delegate()->CreateUserMetricsRecorderProxy());
   if (record_periodic_metrics)
     StartTimer();
 }