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This is a reland of 6252427a.

Original change's description:
> Add WPT tests for feature policy
> 
> 1. Added tests for header policy.
>     a. document.policy shows correctly parsed policy
>     b. local / remote iframes without allow attribute correctly inherit
>        document.policy
>     c. dynamically update allow attribute updates the policy correctly.
> 
> 2. Added tests for nested policies.
> 
> Bug: 732003
> Change-Id: I869449f6bba89fc58997355df27249f403d76808
> Reviewed-on: https://chromium-review.googlesource.com/796952
> Commit-Queue: Luna Lu <loonybear@chromium.org>
> Reviewed-by: Ian Clelland <iclelland@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#531698}

Bug: 732003
Change-Id: I46065efff8c5af2d5279721f3c759580b0807e05
Reviewed-on: https://chromium-review.googlesource.com/887324Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538904}