Add UMA for how long service workers run for.

content/browser/service_worker/embedded_worker_instance.cc

Index: content/browser/service_worker/embedded_worker_instance.cc
diff --git a/content/browser/service_worker/embedded_worker_instance.cc b/content/browser/service_worker/embedded_worker_instance.cc
index 1f1b6e12a4ee7f2ebfeabb301de936ebff20b1a3..727f2d548847080b097b6c2dbad2333621aace02 100644
--- a/content/browser/service_worker/embedded_worker_instance.cc
+++ b/content/browser/service_worker/embedded_worker_instance.cc
@@ -825,6 +825,12 @@ void EmbeddedWorkerInstance::RemoveListener(Listener* listener) {
   listener_list_.RemoveObserver(listener);
 }
 
+void EmbeddedWorkerInstance::SetDevToolsAttached(bool attached) {
+  devtools_attached_ = attached;
+  if (attached)
+    registry_->OnDevToolsAttached(embedded_worker_id_);
+}
+
 void EmbeddedWorkerInstance::OnNetworkAccessedForScriptLoad() {
   starting_phase_ = SCRIPT_DOWNLOADING;
   network_accessed_for_script_ = true;