Allow storage in 1p iframes, regardless of the relative parents of that 1p iframe

[pes10k]

Currently storage is blocked in 3p frames, where 3p-ness is determined relative to the parent frame. So, all the following are true:

3p frames get no storage

https://example.com <-- storage
  <iframe src="//other.com"> <-- no storage

same origin frames get storage

https://example.com <-- storage
  <iframe src="//example.com"> <-- storage
      <iframe src="//other.com"> <-- no storage

frames that have the same origin as the top level document, but are embedded in a 3p frame, also don't get storage.

https://example.com <-- storage
  <iframe src="//other.com"> <-- no storage
      <iframe src="//example.com"> <-- ALSO no storage

This breaks some sites, specially wordpress's dashboard, which has the following pattern

https://yoursite.wordpress.com
  <iframe src="//api.wp.com"> <-- no storage
      <iframe src="//yoursite.wordpress.com"> <-- ALSO no storage

We can unbreak wordpress here by changing the storage rule to being: frames get storage if they're the same eTLD+1 as the top level document, regardless of immediate parent document.

This doesn't add any direct privacy loss, but as a side effect of unbreaking sites, might also re-enable some privacy harming flows.

Related: #9064

[pes10k]

Fixed by @bridiver this is fixed by brave/brave-core#5433

[LaurenWags]

@pes10k @bridiver does this require manual QA? If so, please add QA/Yes label and a test plan. If not, please add QA/No label. Thanks!

[LaurenWags]

Marking as QA/Blocked until there is a test plan.