Make GetServiceWorkerRaceNetworkRequestToken returns optional

A speculative fix for crbug.com/1492640. We're seeing some URL mismatch
inside the custom URLLoaderFactory.

When ServiceWorkerAutoPreload is enabled, URLLoaderFactory being used
from the fetch handler is replaced with the corresponding custom
URLLoaderFactory for the dedupe purpose. This is basically done via the
hash map, which has a token string as a key and URLLoaderFactory as a
value.

The token is set in `service_worker_race_network_request_token_` in the
ResourceRequest class, and the hash map lookup is done in the
LoaderFactoryForWorker::CreateURLLoader. The token is set if the request
is HTTP, however, `service_worker_race_network_request_token_` is not
initialized if the request is not HTTP e.g. extensions, and that allows
the browser to lookup the hash map with uninitialized key, which is not
indended.

This may not be a root cause but we could fix the crash by avoiding the
unexpected hash map lookup.

Bug: 1492640
Change-Id: I1db86bd96102594c227961e40cc39cab9343a90f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4945316
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1211216}