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This enables the detection and parsing of the "Permissions-Policy" HTTP
response header, but does not remove support for the "Feature-Policy"
header. If both headers are returned with a document, then any feature
mentioned in both headers will use the allowlist specified by
Permissions-Policy. A console message will be emitted in that case, to
alert developers to the potential conflict.

Intent-to-Ship thread:
https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAOMQ%2Bw-hVfxO8YYX_WAKNCD96sg_bnFmTDbYDsaL4pfw3s9f4A%40mail.gmail.com

Bug: 1095641

Change-Id: I02f9eb34405bf3c842705bf2c18a14b62fa076e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530793
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826436}