Clean up interface to ProgramImpl::link. We no longer need to pass mutable shaders with the changed to packed varyings. BUG=angleproject:1123 Change-Id: I0a4bbf5d770ff48e10e3de2146840d8987b1e826 Reviewed-on: https://chromium-review.googlesource.com/295194 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>