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

components/metrics/metrics_pref_names.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/metrics/metrics_pref_names.h"
 
 namespace metrics {
 namespace prefs {
 
 // Set once, to the current epoch time, on the first run of chrome on this
@@ skipping 38 matching lines @@
     "user_experience_metrics.reporting_enabled";
 
 // Date/time when the user opted in to UMA and generated the client id for the
 // very first time (local machine time, stored as a 64-bit time_t value).
 const char kMetricsReportingEnabledTimestamp[] =
     "user_experience_metrics.client_id_timestamp";
 
 // 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";
 
 // Number of times the browser has failed to register crash reporting.
 const char kStabilityBreakpadRegistrationFail[] =
     "user_experience_metrics.stability.breakpad_registration_fail";
 
 // Total number of child process crashes (other than renderer / extension
 // renderer ones, and plugin children, which are counted separately) since the
@@ skipping 90 matching lines @@
 // The keys below are strictly increasing counters over the lifetime of
 // a chrome installation. They are (optionally) sent up to the uninstall
 // survey in the event of uninstallation.
 const char kUninstallLaunchCount[] = "uninstall_metrics.launch_count";
 const char kUninstallMetricsPageLoadCount[] =
     "uninstall_metrics.page_load_count";
 const char kUninstallMetricsUptimeSec[] = "uninstall_metrics.uptime_sec";
 
 }  // namespace prefs
 }  // namespace metrics