Use a constant for the number of faces in a cube map. BUG=angle:520 Change-Id: Ia35103059526aff2fc1b29a160d8471f4b5cbd42 Reviewed-on: https://chromium-review.googlesource.com/223107 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>