This provides a cross-platform way to make sure dEQP runs in a low
priority process.

BUG=angleproject:892

Change-Id: I0d12f1eacb78be43edcdb6622f945734c0b377ff
Reviewed-on: https://chromium-review.googlesource.com/273595Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>