Use SkNVRefCnt for a couple common types.

Use SkNVRefCnt for a couple common types.

These types are ref-counted, but don't otherwise need a vtable.
This makes them good candidates for SkNVRefCnt.

Destruction can be a little more direct, and if nothing else,
sizeof(T) will get a little smaller by dropping the vptr.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2232433002

Review-Url: https://codereview.chromium.org/2232433002