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This implements a throttle that will advertise origin policy
support to the server, and will request an policy if necessary.

(This CL doesn't do anything with the manifest yet. That is coming in
 subsequent CLs.)

Gated on --enable-features=OriginManifest.

Bug: 751996
Change-Id: I3ebfa834c29f3eb17568917d1eff83a9261529ac
Reviewed-on: https://chromium-review.googlesource.com/1088918
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Nick Harper <nharper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569582}