[webauthn] Enable usage of BLE Security Keys

This change enables the usage of U2F BLE security keys for the Web
Authentication API. It introduces a new enum representing supported
U2F transport protocols and changes U2fRequest to own the device
discoveries.

Bug: 664630, 763303

Change-Id: Ic2ba03f8bb2e9e097501fe453627c85796d39434
Reviewed-on: https://chromium-review.googlesource.com/887482
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533424}
13 files changed