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..16a9eac86e83a35cb41420cb1581e198990f9ac0 100644
--- a/components/metrics/metrics_pref_names.cc
+++ b/components/metrics/metrics_pref_names.cc
@@ -56,6 +56,10 @@ const char kMetricsReportingEnabledTimestamp[] =
 // The metrics client session ID.
 const char kMetricsSessionID[] = "user_experience_metrics.session_id";
 
+// The last-seen timestamp of setup metrics file.

[grt (UTC plus 2), 2016/02/15 15:42:40]

should this be something like
// The prefix of the last-seen timestamps for persistent histogram files. Values
are named for the files themselves.

[bcwhite, 2016/02/15 19:22:11]

Done.

+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";