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This CL builds the missing integration between the C++ code (starting
from the handler of the PIN requests made by extensions) and the
JavaScript code on the SAML sign-in page:

* dynamically enabling/disabling this UI when the SAML sign-in
  begins/ends;
* sending the PIN request parameters from C++ to JS;
* replying with the entered PIN from JS to C++.

The proper error displaying and other UI improvements will be done in
follow-up CLs.

Bug: 964069
Test: enroll a Chromebook, configure device policies to force-install smart card middleware onto Login Screen, start logging in using a SAML account that uses smart card authentication, check that the PIN dialog appears on top of the SAML page, enter the smart card PIN, check that the login completes
Change-Id: I95c2d5e5110a967ca6b7d8f957bbc8422f62d758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742233Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686151}