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This adds the WebAuthenticationTouchId feature flag (default disabled).
If enabled, the FidoRequestHandler tries to instantiate a
TouchIdAuthenticator on supported platforms for the current request.

Bug: 678128
Change-Id: Iddc4ea00b3247b6ffbe875e280e1bf30488ca81f
Reviewed-on: https://chromium-review.googlesource.com/1064921
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561605}