URLPattern: Escape pattern baseURL values.

This addresses the problem described in this spec issue:

https://github.com/WICG/urlpattern/issues/170

And this spec PR:

https://github.com/WICG/urlpattern/pull/172

Bug: 1380534
Change-Id: I37b5f77e674f8c23ff59e6224d92c7a0d767e7cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4020802


Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071919b}