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At the moment NativeIO can only be used from origins that use HTTPS
or are served locally, but all potentially trustworthy origins (especially
Chrome extensions) should have access to it. Relying on this centrally managed
definition also has the advantage that, as our view of trustworthiness changes,
so does NativeIO's behavior.

Also add a web platform test that confirms that NativeIO is not accessible from
untrustworthy origins.

Bug: 914488
Change-Id: I925a4bf39912c9747c006b1f80bc06a8ec606b23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353452
Commit-Queue: Emanuel Krivoy <krivoy@google.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806167}