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This adds Feature Policy checks to the mojo permission service and
corresponding tests. Because multiple permissions can be requested at the same
time, part of a request can be answered synchronously (by feature policy) and
the other part asynchronously (by quering the PermissionManager). Thus we need
to track the partially answered request in the PendingRequest class.

Feature policy checks are done here (in content) as they will form part of the
specifications for various features and thus it is best if other embedders of
content also benefit from these checks.

BUG=689802
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2874053003
Cr-Commit-Position: refs/heads/master@{#475832}