Test Runner: Really reuse displays with --bot-mode. 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/+/2419638 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>