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Thereby fix signal handling for SIGTERM and SIGINT.

The steps needed to start Xvfb are drawn from:
https://github.com/cgoldberg/xvfbwrapper/blob/master/xvfbwrapper.py
https://github.com/revnode/xvfb-run/blob/master/xvfb-run

Added unit tests to presubmit to test xvfb.py


Bug: 932240
Change-Id: I3b9439991697ae94e98b93e4f1fcfd411a451536
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548424
Commit-Queue: Ilia Samsonov <isamsonov@google.com>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652562}