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As discussed in
https://github.com/w3c/webappsec-fetch-metadata/issues/45,

We have decided to shift the model around nested navigations from
exposure via the request's `mode` to its `destination`. This patch
splits the existing `document` destination into three parts:
 - `document` for top-level navigations and, for the moment, <portals>.
 - `iframe` for <iframe> navigations.
 - `frame` for <frame> navigations.

Subsequent patch will remove the `nested-document` mode when we're ready
to ship `Sec-Fetch-Dest`.

Bug: 1011724
Change-Id: I23f23922fb49523aa050f59cbf13aabc086600bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847295Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Yifan Luo <lyf@google.com>
Cr-Commit-Position: refs/heads/master@{#705527}