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For all successful WebAuthN API transactions, notify observers of
FidoRequestHandler, then AuthenticatorRequestClientDelegate of which
transport protocol was used to talk to the authenticator that was used
to service the request.

This will be saved as Chromium preference, and used to infer which
transport type to default to when user invokes WebAuthN API again.

Bug: 847985
Change-Id: I9ecf27209ec959d740664e34fe088a2e4499e982
Reviewed-on: https://chromium-review.googlesource.com/1175500
Commit-Queue: Jun Choi <hongjunchoi@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583517}