Add startup category to service worker's background tracing config

After crrev.com/c/1832202 renderer startup traces has been recording
correctly under the category "startup." Add the category to service
worker background tracing config so that we can collect these traces.

Bug: 561209
Change-Id: Ic27214be3f328dff47365083d8797dc7f36b3c46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1839278Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702769}

content/browser/tracing/background_tracing_config_impl.cc

@@ -523,8 +523,8 @@ TraceConfig BackgroundTracingConfigImpl::GetConfigForCategoryPreset(
       return TraceConfig(
           "benchmark,toplevel,ipc,base,ServiceWorker,CacheStorage,Blob,"
           "IndexedDB,loading,mojom,navigation,renderer,blink,blink_gc,blink."
-          "user_timing,blink.worker,fonts,disabled-by-default-cpu_profiler,"
-          "disabled-by-default-network",
+          "user_timing,blink.worker,fonts,startup,disabled-by-default-cpu_"
+          "profiler,disabled-by-default-network",
           record_mode);
     case BackgroundTracingConfigImpl::CategoryPreset::BENCHMARK_POWER:
       return TraceConfig(