hide getClipStack

BUG=skia:6122

Change-Id: I0aca03b349541174ced019f4f2116e0015c980c4
Reviewed-on: https://skia-review.googlesource.com/7097
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed