Add UMA for how long service workers run for.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 102d9ce5b5d82f8afcb3baefefce271c0a7216d4..3cedada0ae264b415444c7ed40c00cbe65bb5ab3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -61961,6 +61961,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.Runtime" units="ms">
+  <owner>falken@chromium.org</owner>
+  <summary>
+    The amount of time a service worker ran for (wall time). Recorded when the
+    service worker stops. Not recorded if DevTools was ever attached to the
+    service worker (for this run only; if DevTools is detached and the service
+    worker runs again, the metric can be recorded for that run). See also
+    ServiceWorker.StillRunningTime.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ScriptCachedMetadataSize" units="bytes">
   <owner>horo@chromium.org</owner>
   <summary>
@@ -62128,6 +62139,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.StillRunningTime" units="ms">
+  <owner>falken@chromium.org</owner>
+  <summary>
+    The amount of time a service worker has been running (wall time), if it's
+    been running for over five minutes. Recorded periodically by a timer that
+    fires every five minutes. Whenever a service worker is running, the timer is
+    active and checks all service worker runtimes, recording any that exceed
+    five minutes.
+
+    Therefore the metric is recorded multiple times for the same running worker
+    (i.e., when its runtime exceeds 5, 10, 15, 20, etc minutes).
+
+    Not recorded if DevTools was ever attached to the service worker (for this
+    run only; if DevTools is detached and the service worker runs again, the
+    metric can be recorded for that run). See also ServiceWorker.Runtime.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.StopWorker.Status"
     enum="ServiceWorkerStopStatus">
   <obsolete>