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This CL is a refactor with no intended behavior changes.

The new ScreenManager interface was landed in M88. Ash beta is now M88,
and Lacros is not available on Ash stable M87, so Lacros can always
assume that the new ScreenManager interfaces exist.

Removing this dead code is a prerequisite to refactoring the test code,
which itself is a prerequisite for other lacros_chrome_browsertests.

Change-Id: Ia5311010c8003f763409b2dc556a7dd6a99fc80d
Bug: 1155662, 1156872, 1094460
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580107Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835254}