ServiceWorker: Add some tracing to installed script loading.

Change-Id: I4d2a85675a73a09f48e94ec504e089621ca0ede3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541896
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646341}

third_party/blink/renderer/modules/service_worker/service_worker_installed_scripts_manager.cc

@@ -267,6 +267,9 @@ bool ServiceWorkerInstalledScriptsManager::IsScriptInstalled(
 std::unique_ptr<InstalledScriptsManager::ScriptData>
 ServiceWorkerInstalledScriptsManager::GetScriptData(const KURL& script_url) {
   DCHECK(!IsMainThread());
+  TRACE_EVENT1("ServiceWorker",
+               "ServiceWorkerInstalledScriptsManager::GetScriptData", "url",
+               script_url.GetString().Utf8().data());
   if (!IsScriptInstalled(script_url))
     return nullptr;