GitLab

* Show overlay for the 3rd-party SAML page.
* Hide bottom buttons for the 3rd-party SAML page.

Unfortunately my refactoring CL:2046174 is still in-flight. So I
followed the model from CL:2000938 in the meantime. As this is needed
for M82.

Fixed: 1001281
Change-Id: Ia8f2d8cbabb2f44e387ad5f76b6a83997654ae9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089734Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747307}