Prevent fallback on default branch for case EbtSamplerCubeShadow BUG=angleproject:1549 Change-Id: I8f003257b6f39282cb52ccdbacb73860ab37c919 Reviewed-on: https://chromium-review.googlesource.com/390271 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>