Add UMA for how long service workers run for.

content/browser/service_worker/service_worker_metrics.cc

Index: content/browser/service_worker/service_worker_metrics.cc
diff --git a/content/browser/service_worker/service_worker_metrics.cc b/content/browser/service_worker/service_worker_metrics.cc
index aa0aa0862133853c59e79b0674dfe8427ddb54a9..ab94115d23c36f0ebed703b573547cdb1d79ccbb 100644
--- a/content/browser/service_worker/service_worker_metrics.cc
+++ b/content/browser/service_worker/service_worker_metrics.cc
@@ -978,4 +978,12 @@ void ServiceWorkerMetrics::RecordContextRequestHandlerStatus(
   }
 }
 
+void ServiceWorkerMetrics::RecordRuntime(base::TimeDelta time) {
+  UMA_HISTOGRAM_LONG_TIMES("ServiceWorker.Runtime", time);

[nhiroki, 2017/02/23 07:54:23]

According to a header comment[1], UMA_HISTOGRAM_LONG_TIMES can record duration
up to an hour. I feel it'd be somewhat short. WDYT?

[1]
https://chromium.googlesource.com/chromium/src/+/dbfe909f3404ff148339c0396556...

[falken, 2017/02/23 08:18:18]

Yes I saw this but figured for our purposes 1 hour is not much different from a
1 day, also I figure the buckets would get more coarse even though we would like
to distinguish between shorter times (e.g., 1 sec vs 10 sec vs 1 minute is
pretty significant). Not totally sure. WDYT?

[nhiroki, 2017/02/23 09:05:05]

I see, then UMA_HISTOGRAM_LONG_TIMES would be appropriate. Thank you for the
clarification.

+}
+
+void ServiceWorkerMetrics::RecordStillRunning(base::TimeDelta time) {
+  UMA_HISTOGRAM_LONG_TIMES("ServiceWorker.StillRunningTime", time);
+}
+
 }  // namespace content