[FedCM] Allow mismatching trailing slash for provider urls

The provider url from the API call must match the one in the manifest
list by design.

However, it's possible for developers to append a trailing slash in one
of them but not in the other one especially when there's path involved.
Besides, for GURL without path, |provider_.spec()| will append a
trailing slash automatically. Therefore we relax the requirement by
allowing mismatch on trailing slash.

Bug: 1316823
Change-Id: If28c8d94209a990f6136417dd6efb61ee2ba3967
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3589174
Reviewed-by: Sam Goto <goto@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#993368}

• content/browser/webid/federated_auth_request_impl.cc[diff]
• content/browser/webid/federated_auth_request_impl_unittest.cc[diff]