[Locked Fullscreen] Disable Assistant in locked mode

Locked fullscreen mode is a special fullscreen mode used for administering
quizzes to students on Chrome OS devices (where the user is basically locked
inside of one browser window).

Assistant is breaking the mode since it allows students to lookup answers to
questions.  Fix this by disabling the Assistant inside the mode.

Bug: 952037
Test: locally build and test with lock screen mode on.
Change-Id: I1b9e539374346f247d15ad65e7d11bfe35b4405b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616942
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Ivan Ĺ andrk <isandrk@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661352}
18 files changed