webauthn: disable platform authenticators in incognito mode

This changes IsUserVerifyingPlatformAuthenticatorAvailable to return
false in incognito mode, and disable platform authenticator
instantiation for MakeCredential/GetAssertion in incognito.

Also change IsUVPAA to not return true on platforms where Touch ID is
enabled but the embedder does not not provide a configuration.

Bug: 678128
Change-Id: I2fc6b0182fcb9ae718acd842f1247baee81c5281
Reviewed-on: https://chromium-review.googlesource.com/1149115
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578535}
3 files changed