Add Startup.SystemUptime histogram.

components/startup_metric_utils/browser/startup_metric_utils.cc

Index: components/startup_metric_utils/browser/startup_metric_utils.cc
diff --git a/components/startup_metric_utils/browser/startup_metric_utils.cc b/components/startup_metric_utils/browser/startup_metric_utils.cc
index 76491ff103f3c6d27857a6953fb17682105bc694..c65c078e69f18aeb7e2966195af09708a05f3ba4 100644
--- a/components/startup_metric_utils/browser/startup_metric_utils.cc
+++ b/components/startup_metric_utils/browser/startup_metric_utils.cc
@@ -132,6 +132,21 @@ typedef NTSTATUS (WINAPI *NtQuerySystemInformationPtr)(
         g_startup_temperature);                                               \
   }
 
+// Returns the time elapsed between system boot and process launch.
+base::TimeDelta GetStartupUptime() {

[gab, 2016/01/05 15:10:00]

GetSystemUptimeOnProcessLaunch() ?

[fdoray, 2016/01/05 15:34:01]

Done.

+  DCHECK(!g_process_creation_ticks.Get().is_null());
+  // base::SysInfo::Uptime returns the time elapsed between system boot and now.
+  // Substract the time elapsed between process launch and now to get the time
+  // elapsed between system boot and process launch.
+  return base::SysInfo::Uptime() -
+         (base::TimeTicks::Now() - g_process_creation_ticks.Get());
+}
+
+void RecordUptimeHistogram() {
+  UMA_HISTOGRAM_WITH_STARTUP_TEMPERATURE(UMA_HISTOGRAM_LONG_TIMES_100,
+                                         "Startup.Uptime", GetStartupUptime());

[gab, 2016/01/05 15:10:00]

"Startup.SystemUptime" ? (i.e. to make it clear it's the system's uptime)

[fdoray, 2016/01/05 15:34:01]

Done.

+}
+
 // On Windows, records the number of hard-faults that have occurred in the
 // current chrome.exe process since it was started. This is a nop on other
 // platforms.
@@ -392,6 +407,7 @@ void RecordBrowserMainMessageLoopStart(const base::TimeTicks& ticks,
                                        bool is_first_run) {
   AddStartupEventsForTelemetry();
   RecordHardFaultHistogram(is_first_run);
+  RecordUptimeHistogram();
   RecordMainEntryTimeHistogram();
 
   const base::TimeTicks& process_creation_ticks =