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Currently, |AuthenticatorImpl| is binding
|CreatePlatformAuthenticatorIfAvailable| with |base::Unretained| for the
|MaybeAddPlatformAuthenticatorCallback| argument to the
|FidoRequestHandlerBase| ctor. The callback was originally invoked
synchronously from the ctor, but then recently changed to async
invocation to account for observer registration (crrev.com/c/1175418).

This is a potential callback lifetime issue because the
AuthenticatorImpl and its request handler can now theoretically get
destroyed before the callback gets invoked.

To work around this issue, replace the callback with a method on the
request handler invoked by the AuthenticatorImpl.

Bug: 875338
Change-Id: I089f0783a46791ae8d9d6bac5f0e663e52075b86
Reviewed-on: https://chromium-review.googlesource.com/1179081
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584288}