Hook WebAuthN to //device/u2f to perform credential registration.

This change implements the browser-side of authenticator.mojom to
build and send a U2F-style request to //device/u2f to talk to a
hardware authenticator. This also adds a utility class to map the u2f
response from U2fDevice to a CTAP response per the FIDO2 and WebAuthN specs.

Bug: 664630
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I7d5f9e2308c45fdd8ca9463a8a78c2f88d9aeb80
Reviewed-on: https://chromium-review.googlesource.com/710516


Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518277}