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A cross origin isolated page is a page that has set COOP and COEP,
eventually unlocking access to powerful APIs such as SharedArrayBuffer.

This first patch goal is to introduce cross origin isolated
BrowsingInstances that only contain cross origin isolated pages. It
modifies the SiteInstance selection process to make sure the invariant
holds. In more details:

- BrowsingInstances now have a is_coop_coep_cross_origin_isolated
  boolean, that indicates if all the pages contained into it have been
  COOP+COEP enabled. Each BrowsingInstance also holds the origin of the
  top level pages contained in it, unique across all the top level pages
  in it.

- CHECKS and DCHECKS enforce the BrowsingInstance invariant.

See the full design doc
https://docs.google.com/document/d/1OFaz1Txi4ynFLmRqNTLFF3qd6jm4kK4GkJdmgr5_aZA/edit?usp=sharing
for more information.

It was based on clamy@ work here:
https://chromium-review.googlesource.com/c/chromium/src/+/2066844
and further discussed here:
https://chromium-review.googlesource.com/c/chromium/src/+/2119854

BUG=1055779

Change-Id: I4829337f5fd4a4c2cd18f2cfaaf7ef8054eb67a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2230632Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Arthur Hemery <ahemery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800556}