The prior design didn't pass the correct flag to the subprocess.
Instead add a new --reuse-displays flag that forces display reuse
on in ANGLE tests. Pass the new flag to the child processes when
running with --bot-mode.

Bug: angleproject:3162
Change-Id: I5f62125d83d339b5e8b2506b4ca4656976f7398e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419638Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>