FramebufferNULL::readPixels: write pixels for tests BUG=602737 Change-Id: I8c24985358dcd297cb437c501b7a3944e36d98de Reviewed-on: https://chromium-review.googlesource.com/644210 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>