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This changes TouchIdAuthenticator::IsAvailable to check whether the
current executable is signed with a keychain-access-groups entitlement
matching the value that the authenticator is instantiated with. Without
it, calls to the keychain API to access credentials will fail.

Some embedders seem to provide a TouchIdAuthenticatorConfig that
attempts to configure Touch ID but don't entitle their binary as
required. This results in Touch ID being available but
MakeCredential/GetAssertion calls hanging until they time out. With this
change, Touch ID will simply be unvavailable instead.

Bug: 898577
Change-Id: I7b653a492661f36c921ab4fcd8785d90c92612ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628734
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664837}