Use a "gyp trick" to define a default value for use_x11, thanks goes to
kbr for remembering the other instance of this trick.

BUG=angleproject:892

Change-Id: Ifb64af4e56d9eb90993760a495008d95dd31ae3d
Reviewed-on: https://chromium-review.googlesource.com/278158Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>