Add U2F legacy check and notification.

This checks whether the legacy U2F feature is enabled and notifies the
user about a security vulnerability.

BUG=chromium:967396
TEST=Manual

Change-Id: I24a227a40ececa2ba533dd6d825e521422bb6fe0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634706
Commit-Queue: Mattias Nissler <[email protected]>
Reviewed-by: Dan Erat <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Cr-Commit-Position: refs/heads/master@{#669191}
10 files changed