Type-erase SkAutoMutexAcquire and SkAutoExclusive.

This is purely for convenience, to not need to write the lock type
in the guard anymore.  This should all inline away.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2055023003

Review-Url: https://codereview.chromium.org/2055023003
4 files changed