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The IsUserVerifyingPlatformAuthenticator() WebAuthn API call needlessly
required the content embedder to provide their own implementation in an
AuthenticatorRequestClientDelegate specialization. This change moves the
ChromeAuthenticatorRequestDelegate implementation into the content layer
(AuthenticatorCommon). The implementation is general enough to be useful
for other embedders and it doesn't depend on any state internal to the
delegate.

Concretly, IsUVPAA()
- on Windows returns the corresponding value from the native WebAuthn
API;
- on macOS reflects TouchIdAuthenticator availability as long as the
embedder provides an implementation of GetTouchIdAuthenticatorConfig.

Also collapse methods in AuthenticatorCommon that determine which
transports to instantiate discoveries for, and also add a safeguard
against instantiating platform discoveries in contexts where IsUVPAA()
returns false.

Change-Id: I8eb68ca6f8cd17fae61d9d0294b872ef0cfb3465
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083936
Commit-Queue: Martin Kreichgauer <martinkr@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748544}