GitLab

This adds a fake implementation of WinWebAuthnApi along with a scoped
override of WinWebAuthnApi::GetDefault, as well as a test to verify USB
authenticator instantiation depending on API availability and feature
flag state.

Bug: 898718
Change-Id: Iff3a317b4540986775a40fc872754ba0571ae88a
Reviewed-on: https://chromium-review.googlesource.com/c/1316839
Commit-Queue: Martin Kreichgauer <martinkr@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606254}