service worker: Attribute purpose to start worker attempts for UMA

content/browser/service_worker/service_worker_context_wrapper.cc

Index: content/browser/service_worker/service_worker_context_wrapper.cc
diff --git a/content/browser/service_worker/service_worker_context_wrapper.cc b/content/browser/service_worker/service_worker_context_wrapper.cc
index 770a78a054a5244a9e88bb2d28636eb56b38e516..cba52e97f31a6203d3a465709efc4d9e7b6d2ef1 100644
--- a/content/browser/service_worker/service_worker_context_wrapper.cc
+++ b/content/browser/service_worker/service_worker_context_wrapper.cc
@@ -62,6 +62,7 @@ void StartActiveWorkerOnIO(
     // it from being deleted while starting the worker. If the refcount of
     // |registration| is 1, it will be deleted after WorkerStarted is called.
     registration->active_version()->StartWorker(
+        ServiceWorkerMetrics::EventType::UNKNOWN,
         base::Bind(WorkerStarted, callback));
     return;
   }