[WPT/referrer-policy] Support subresource requests from data: URL workers

This CL makes stripUrlForUseAsReferrer() more spec-conformant,
in order to support `data:` URLs as the argument,
which can happen in testing subresource requests from
data: URL workers (no such tests so far though).

Bug: 906850
Change-Id: I3ba7b1fbf27e79624d9c09fa2c65862b45d17338
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102302
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750490}