SK_DECLARE_STATIC_MUTEX -> static SkMutex

There's no need to use a macro to declare static SkMutexes any more
(and there's likewise no need to restrict them to global scope).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1948193002

Review-Url: https://codereview.chromium.org/1948193002
23 files changed