Added UMA metrics for the system update UI in the status area.

ash/common/system/update/tray_update.cc

Index: ash/common/system/update/tray_update.cc
diff --git a/ash/common/system/update/tray_update.cc b/ash/common/system/update/tray_update.cc
index e72bd809078d22424542e2573ceebe0edca56946..5da745c35d206208bff6156889bbf4d24a4ce669 100644
--- a/ash/common/system/update/tray_update.cc
+++ b/ash/common/system/update/tray_update.cc
@@ -4,6 +4,7 @@
 
 #include "ash/common/system/update/tray_update.h"
 
+#include "ash/common/metrics/user_metrics_action.h"
 #include "ash/common/system/tray/fixed_sized_image_view.h"
 #include "ash/common/system/tray/system_tray_delegate.h"
 #include "ash/common/system/tray/system_tray_notifier.h"
@@ -74,6 +75,8 @@ class UpdateView : public ActionableView {
   // Overridden from ActionableView.
   bool PerformAction(const ui::Event& event) override {
     WmShell::Get()->system_tray_delegate()->RequestRestartForUpdate();
+    WmShell::Get()->RecordUserMetricsAction(
+        UMA_STATUS_AREA_OS_UPDATE_DEFAULT_SELECTED);
     return true;
   }