hide index8 support

Bug: skia:
Change-Id: I4ab7f4665cc9992319d3c9a5e02a417d503c189e
Reviewed-on: https://skia-review.googlesource.com/20121
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed