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This CL prepares the profile picker to work with the force signin
policy. The list of changes:

- Propagate IsSigninRequired() value from ProfileAttributesEntry to the
  WebUI
- Modify the profile card to display "Sign in [lock]" prompt on profiles
  that require sign-in
- Remove asserts checking that the force signin policy is disabled
- Add native handlers for launching a locked profile and creating a new
  profile when the force signin policy is enabled

Screenshot:
https://drive.google.com/file/d/1Y03Kpe_4CoO5DWLbJ79Fu5j-hjjz0xzz/view?usp=sharing

Bug: 1156096
Change-Id: I8aa315c4459ecf3e9d0faed331a3590aa92a0169
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584002
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Monica Basta <msalama@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837124}