GitLab

This is a first pass at implementing a header-based version of origin
isolation, alongside our existing origin policy-based one, per [1]. It
does not yet parse the header, instead simply using its presence or
absence. This CL includes browser tests only for now; web platform tests
will be done in a follow-up.

[1]: https://github.com/WICG/origin-isolation/commit/1c16647387b6be1bb170ed8ce24a5bd179abfa98

Bug: 1042415
Change-Id: Iaaa97a87e166aeedac2313f718322e5cdb7462e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2113152
Commit-Queue: Domenic Denicola <domenic@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756370}