SkValue: SkXfermode

Implement:
  template<> SkValue SkToValue<SkXfermode>(const SkXfermode*);

  template<> bool SkFromValue<SkXfermode*>(const SkValue&, SkXfermode**);

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1585813004

NOTRY=true

Review URL: https://codereview.chromium.org/1585813004
15 files changed