Alter SkXfermode's asFragmentProcessor & asXPFactory contracts

TBR=bsalomon@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1674673002

Review URL: https://codereview.chromium.org/1674673002
12 files changed