This patch uses the new Feature Policy framework to control access to
WebUSB from embedded frames. When Feature Policy is available WebUSB's
default policy ["self"] is enforced. When it isn't available WebUSB is
simply disallowed in embedded frames.

A follow-up patch will remove the parser for the "allowed origins"
descriptor as they are no longer used and have been removed from the
specification.

BUG=711443

Review-Url: https://codereview.chromium.org/2815003005
Cr-Commit-Position: refs/heads/master@{#471487}