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Standard functions for people to check if content from an origin can be
considered to have been transferred to the browser securely, as defined in
https://www.w3.org/TR/powerful-features/#is-origin-trustworthy.

BUG=362214,470142

Review URL: https://codereview.chromium.org/1049533002

Cr-Commit-Position: refs/heads/master@{#325495}