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This CL adds SigninFragmentBase that implements generic screen for
sign-in and unified consent. The view for this fragment will be added by
subsequent CLs. This CL also adds SigninFragment that extends
SigninFragmentBase for SigninActivity.

Bug: 814728
Change-Id: Ie4081150a31f67bb00868f49300414eee05f3f83
Reviewed-on: https://chromium-review.googlesource.com/961201Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543374}