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..6eb58fa8a67903a926b5da5d21c82615fc0d59ab 100644
--- a/components/metrics/metrics_pref_names.cc
+++ b/components/metrics/metrics_pref_names.cc
@@ -56,6 +56,11 @@ const char kMetricsReportingEnabledTimestamp[] =
 // The metrics client session ID.
 const char kMetricsSessionID[] = "user_experience_metrics.session_id";
 
+// The prefix of the last-seen timestamp for persistent histogram files.
+// Values are named for the files themselves.
+const char kMetricsLastSeenPrefix[] =
+    "user_experience_metrics.last_seen.";
+
 // Number of times the browser has been able to register crash reporting.
 const char kStabilityBreakpadRegistrationSuccess[] =
     "user_experience_metrics.stability.breakpad_registration_ok";