Support GL_NV_fence in the NULL backend. BUG=770696 Change-Id: Ice5345eddc4cec61cd5ce33a7cd5d45fa68379a5 Reviewed-on: https://chromium-review.googlesource.com/695918 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>