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The results of client extensions, after some discussion, have settled on
being a dictionary returned by a callback on the
|PublicKeyCredential|[1].

This change implements that callback and plumbs through the results of
the |appid| extension[2], since that's the only extension that we
currently implement.

[1] https://w3c.github.io/webauthn/#iface-pkcredential
[2] https://w3c.github.io/webauthn/#sctn-appid-extension

Bug: 818303
Change-Id: Ie7963229cc74fa209cb1c61d00e0ff63e361ace3
Reviewed-on: https://chromium-review.googlesource.com/946732Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541257}