fido: reintroduce the WebAuthn caBLE feature and UI flag

This adds the WebAuthenticationCable feature flag and related
enable-web-authentication-cable-support UI flag. These flags were
previously removed in 800d018cf6d2a00c4d03ce57ed12f1debe06bc2d, but we
are anticipating having to disable caBLE on some OSes in the future based on
user feedback.

Also clean up some ununsed flag_descriptions entries and a dangling
declaration of the WebAuthenticationCableWin flag.

Bug: 924800
Change-Id: I08623b19c62b894391e903f7dfce30f0aa00b7f2
Reviewed-on: https://chromium-review.googlesource.com/c/1433133
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#625522}
8 files changed