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This fixes an error in the inherited policy calculation where a frame
which was explicitly allowed to use a feature through the Feature-Policy
header would be blocked if the "allow" attribute was also present on
the iframe element.

The behaviour now matches the spec algorithm at
https://wicg.github.io/feature-policy/#define-inherited-policy-in-container

Tests have been added to catch this case.

Change-Id: I7a394202c615be3088fda738d7fc65f16bcadf34
Reviewed-on: https://chromium-review.googlesource.com/c/1293610Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605431}