change pixelref to not inherit from SkFlattenable

If I can "inline" MallocPixelRef unflatten, then I think we can delete this code. The only caller today should be unflattening in the legacy path for bitmaps.

R=robertphillips@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/320873003
11 files changed