Implement onAppendStages for SkPictureShader

BUG=skia:

Change-Id: Ibc525a666f17fed1d987c0316b1963ebed7de343
Reviewed-on: https://skia-review.googlesource.com/7739
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
2 files changed