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
diff --git a/tests/RefCntTest.cpp b/tests/RefCntTest.cpp
index a9b461f..3691593 100644
--- a/tests/RefCntTest.cpp
+++ b/tests/RefCntTest.cpp
@@ -71,7 +71,7 @@
     thing4.join();
 
     REPORTER_ASSERT(reporter, ref->unique());
-    REPORTER_ASSERT(reporter, ref->getWeakCnt() == 1);
+    SkDEBUGCODE(REPORTER_ASSERT(reporter, ref->getWeakCnt() == 1));
     ref->unref();
 }