Add UMA for how long service workers run for.

content/browser/service_worker/embedded_worker_registry.cc

Index: content/browser/service_worker/embedded_worker_registry.cc
diff --git a/content/browser/service_worker/embedded_worker_registry.cc b/content/browser/service_worker/embedded_worker_registry.cc
index 88a24dd2acf3ce1970b6b592a4603ea74b8e1f37..e8c0c86c16d922aca7384566b4e94ed64be672e9 100644
--- a/content/browser/service_worker/embedded_worker_registry.cc
+++ b/content/browser/service_worker/embedded_worker_registry.cc
@@ -140,6 +140,7 @@ void EmbeddedWorkerRegistry::OnWorkerStarted(
   }
 
   worker->OnStarted();
+  lifetime_tracker_.StartTiming(embedded_worker_id);
 }
 
 void EmbeddedWorkerRegistry::OnWorkerStopped(
@@ -150,6 +151,7 @@ void EmbeddedWorkerRegistry::OnWorkerStopped(
     return;
   worker_process_map_[process_id].erase(embedded_worker_id);
   worker->OnStopped();
+  lifetime_tracker_.StopTiming(embedded_worker_id);
 }
 
 void EmbeddedWorkerRegistry::OnReportException(
@@ -179,6 +181,10 @@ void EmbeddedWorkerRegistry::OnReportConsoleMessage(
                                  line_number, source_url);
 }
 
+void EmbeddedWorkerRegistry::OnDevToolsAttached(int embedded_worker_id) {
+  lifetime_tracker_.AbortTiming(embedded_worker_id);
+}
+
 void EmbeddedWorkerRegistry::RemoveProcess(int process_id) {
   std::map<int, std::set<int> >::iterator found =
       worker_process_map_.find(process_id);
@@ -269,6 +275,7 @@ void EmbeddedWorkerRegistry::DetachWorker(int process_id,
   worker_process_map_[process_id].erase(embedded_worker_id);
   if (worker_process_map_[process_id].empty())
     worker_process_map_.erase(process_id);
+  lifetime_tracker_.StopTiming(embedded_worker_id);
 }
 
 EmbeddedWorkerInstance* EmbeddedWorkerRegistry::GetWorkerForMessage(