Handle loading multiple non-http(s) schemes from ServiceWorkerScriptLoaderFactory.

Taken from the TODO comment:
  // It's assumed that the non-network factory can handle any non-http(s) URL
  // that this service worker may load.
  // TODO(falken): We probably need to pass the whole (scheme -> factory) map
  // instead of just giving a single factory, since it's conceivable a service
  // worker can import scripts from multiple schemes:
  // importScripts(['scheme1://blah', 'scheme2://blah', 'scheme3://blah']).
  // So far it's not needed with the Chrome embedder since it's only used
  // for chrome-extension:// URLs.