[webauthn] Resident support for AddCredential command

Add support for injecting resident credentials via the DevTools API. This was
missed in the original implementation. Resident Credentials need to store the
Relying Party ID and User Handle, so these two parameters are added. The
Relying Party ID hash is made redundant and therefore removed.

This is one in a series of patches intended to create a Testing API for
WebAuthn, for use in Web Platform Tests and by external webauthn tests.

For an overview of overall design, please see
https://docs.google.com/document/d/1bp2cMgjm2HSpvL9-WsJoIQMsBi1oKGQY6CvWD-9WmIQ/edit?usp=sharing

Bug: 922572
Change-Id: Ifee5edb0150593238e425fc3b2963ed04dda1609
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728423
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683281}