SkValue: improve SkFromValue<T> implementation

Add template<> bool SkFromValue<float>(const SkValue&, float*);
Refactor template<> bool SkFromValue<SkMatrix>(const SkValue&, SkMatrix*);

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1605093003

Review URL: https://codereview.chromium.org/1605093003
2 files changed