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This CL does two things:
1. Create a lacros_chrome_browsertests target for tests that live at
   lacros side and exercises crosapis.
2. Added a sample test that exercises screen manager apis to serve as
   proof of concept and demonstration.

Bug: 1120582
Change-Id: I010cdb786e84d3f4a18c00cb3120f15de2fbc6ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2408877
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807201}