Convert SkRefCnt to std::atomic.

This enables removing the more complicated atomic shims from SkAtomics.h.

TBR=reed
This doesn't actually change any API.

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN-Trybot,Test-Ubuntu-GCC-Golo-GPU-GT610-x86_64-Release-TSAN-Trybot

Review URL: https://codereview.chromium.org/1867863002
9 files changed