In the current implementation, top-level service worker script is fetched from the main thread via WorkerClassicScriptLoader (see WebEmbeddedWorkerImpl::OnShadowPageInitialized()). To avoid dependencies on the main thread, that should be done from the worker thread.

Implementation plan
1) Add a runtime flag like OffMainThreadServiceWorkerScriptFetch, and wrap following steps with flag checks.
2) In WebEmbeddedWorkerImpl::OnShadowPageInitialize(), bypass WorkerClassicScriptLoader creation, and call StartWorkerThread() like the mojom::ScriptType::kModule case.
3) In WebEmbeddedWorkerImpl::StartWorkerThread(), call WorkerThread::ImportClassicScript() instead of WorkerThread::EvaluateClassicScript(). A worker script will be fetched and evaluated on the worker thread.

Testing plan
- Run web_tests with the OffMainThreadServiceWorkerScriptFetch flag. For references, see "omt-worker-fetch" in VirtualTestSuites ("omt-worker-fetch" is the test setting for off-the-main-thread dedicated worker script fetch).

References
- dedicated_worker.cc does the similar thing for off-the-main-thread dedicated worker script fetch. Search the code by RuntimeEnabledFeatures::OffMainThreadWorkerScriptFetchEnabled.