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Use Polymer data binding in order to compute elements' visibility on the
screen_gaia_signin screen.

This removes most of the imperative manipulation of "hidden" attributes
and other related logic from the JavaScript code.

The only remaining old-style visibility logic is the one for SAML video
recording (saml-notice-recording-indicator) - that one will be
refactored in a follow-up CL.

Bug: 964069
Test: manual
Change-Id: If96b65dd6fb3664b66125b162e09e5d05a7e8dba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627383
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664465}