webauthn: apply De Morgan's to simplify logic.

Change-Id: I58c55695d01e7630977c4e901dea6587989040b0
Bug: none
Reviewed-on: https://chromium-review.googlesource.com/920901


Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537931}