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Add the `RemoveCredential` WebAuthn command to ChromeDriver.

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: I3a547e5cc63af1099327606ccf22e8f916ea34d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764634
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689557}