This will allow us to speed up test execution by using multiple
processes on non-Windows platforms. Because of the process isolation
of "bot mode" we don't run into display re-use bugs like we do when
running in a single process.

Bug: angleproject:3162
Change-Id: I25370d4a07236e34f87e1c1efef8684f9891ecf8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2407835Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>