Gyp build support for Chrome OS. Don't hard code "chromeos" to zero. Fix incorrect assumptions that X11 would always be used on linux. Introduce "angle_use_drm" as the Chrome OS alternative to "angle_use_x11." Define USE_OZONE for Chrome OS. Allow overriding pkg-config. BUG=angleproject:1297 Change-Id: Ie85b5c37f39170e0d11f8fc1bb70c175db6ee334 Reviewed-on: https://chromium-review.googlesource.com/323610 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>