device/fido/mac: hide DeleteWebAuthnCredentials behind flag

This changes device::fido::mac::DeleteWebAuthnCredentials to have no
effect when the feature flag for the Touch ID authenticator
(WebAuthenticationTouchId) is turned off.

Bug: 678128
Change-Id: I2fef6fe53903dad3329a075aa00d5152365af133
Reviewed-on: https://chromium-review.googlesource.com/1129779
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#573832}
3 files changed