BUILD.gn: prepare dEQP test support on Mac BUG=angleproject:1569 Change-Id: I703f4739726c849950ba103049fa39498a4ff1df Reviewed-on: https://chromium-review.googlesource.com/744361 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>