use main frame url for webrequest first party url

[diracdeltas]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan: covered by automated test
fix #4137
fix #4300

auditors: @bridiver

[bridiver]

this looks good, but can we add some tests for it? The changes look right, but it's hard to know for sure without either automated or manual testing

[diracdeltas]

@bridiver thanks, i have rebased and added a test which fails on master but works with this PR. the problem appears to be that firstPartyUrl in an cross-origin iframe is data: instead of the main frame URL, which is consistent with https://tools.ietf.org/html/draft-west-first-party-cookies-07#section-2.1.