Add async tracing to SWVersion::AddRequest.

content/browser/service_worker/service_worker_metrics.h

Index: content/browser/service_worker/service_worker_metrics.h
diff --git a/content/browser/service_worker/service_worker_metrics.h b/content/browser/service_worker/service_worker_metrics.h
index eec21d0699979a2f8a1b4a0e1eb7e004e80be3c1..2827bec9f13d3d0ddd25e0f9697f2e964e5012f2 100644
--- a/content/browser/service_worker/service_worker_metrics.h
+++ b/content/browser/service_worker/service_worker_metrics.h
@@ -91,6 +91,9 @@ class ServiceWorkerMetrics {
   // Used for UMA. Append only.
   enum class Site { OTHER, NEW_TAB_PAGE, NUM_TYPES };
 
+  // Converts an event type to a string. Used for tracing.
+  static const char* EventTypeToString(EventType event_type);
+
   // Excludes NTP scope from UMA for now as it tends to dominate the stats and
   // makes the results largely skewed. Some metrics don't follow this policy
   // and hence don't call this function.