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Adding an aemu_target.py that supports running headless AEMU with swiftshader.
Adds AEMU checkout to the build (with aemu_checkout set to True).
Add support for AEMU in layout tests.

R=marshallk@google.com

Bug: 1000906
Change-Id: Ie4a9e7957c5a33822dd44e8892c0820307ea5b02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789849Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#700484}