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This adds an |is_u2f_only_| field to CtapMakeCredentialRequest to make
authenticators aware whether the current request originates from the
cryptotoken (U2F) extensions. If it does, request handling changes as follows:
 - For Windows API requests, U2F-only mode is enabled.
 - FidoDeviceAuthenticators will only communicate via U2F, not CTAP.

This is done to force the authenticator return a U2F-format attestation
statement, rather than one in an (incompatible) CTAP2 format such as
"packed".

GetAssertionTask already implements a U2F fallback for all requests with an App
ID extension (and CTAP need not be strictly avoided as is the case with
MakeCredential); thus no separate boolean field is required for
CtapGetAssertionRequest.

Bug: 898718
Change-Id: I8f17655d8df530546b62e2237c73aaa997483060
Reviewed-on: https://chromium-review.googlesource.com/c/1338952
Commit-Queue: Kim Paulhamus <kpaulhamus@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609618}