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Add plumbing that allows to skip the "manual password entry"
logic when the smart card SAML sign-in is used. Without this,
the user would be asked to type their password at the end of
their HTTPS-client-certificate-based SAML sign-in.

For this, the gaia sign-in screen JS code will ask the C++ handler
whether the user is expected to have a password. In this CL, the
handler is just a stub. It will be replaced later with an actual
smart card sign-in logic, once it's implemented.

No changes in behavior are expected from this CL alone.

BUG=chromium:826417
TEST=none (existing tests)

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I44012f02fade8823556bcf1200a122b0791d8292
Reviewed-on: https://chromium-review.googlesource.com/1097406Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567203}