Persist setup metrics and have Chrome report them during UMA upload.

components/metrics/metrics_pref_names.cc

Index: components/metrics/metrics_pref_names.cc
diff --git a/components/metrics/metrics_pref_names.cc b/components/metrics/metrics_pref_names.cc
index a5ad6a521df6a25cd609b729b87efef3675a68b5..725c822ff1d09514d6839dfae301b736448ccfc3 100644
--- a/components/metrics/metrics_pref_names.cc
+++ b/components/metrics/metrics_pref_names.cc
@@ -56,6 +56,9 @@ const char kMetricsReportingEnabledTimestamp[] =
 // The metrics client session ID.
 const char kMetricsSessionID[] = "user_experience_metrics.session_id";
 
+// The last-seen timestamp of setup metrics file.
+const char kSetupMetricsLastSeen[] = "user_experience_metrics.setup_last_seen";
+
 // Number of times the browser has been able to register crash reporting.
 const char kStabilityBreakpadRegistrationSuccess[] =
     "user_experience_metrics.stability.breakpad_registration_ok";