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Simplify the TestScreen to use display::ScreenBase.
Always use the global Screen::GetScreen().

The eliminates one WindowSizer constructor and the |screen_| member,
such that the code in test is similar to the code in production.

Bug: 846736
Test: unit_tests
Change-Id: I39121a9b8ae1283c4bbea6194c528450243af24e
Reviewed-on: https://chromium-review.googlesource.com/1073961Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562060}