sk_sp: Covariant Move Constructor and Move Assignment

Also Covariant Copy Constructor, Copy Assignment,
and comparison operators.

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1752093002

Review URL: https://codereview.chromium.org/1752093002
2 files changed