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/295194Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>