GitLab

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}