Factor code between PixelUnpackState and PixelPackState BUG=angleproject:1512 Change-Id: I4c60472d216bfc5198e635d70fd197a5738dde98 Reviewed-on: https://chromium-review.googlesource.com/390133 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>