Convert SkRefCnt to std::atomic.

tests/RefCntTest.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkRefCnt.h"
 #include "SkThreadUtils.h"
 #include "SkTypes.h"
@@ skipping 53 matching lines @@
     SkASSERT(thing2.start());
     SkASSERT(thing3.start());
     SkASSERT(thing4.start());
 
     thing1.join();
     thing2.join();
     thing3.join();
     thing4.join();
 
     REPORTER_ASSERT(reporter, ref->unique());
-    REPORTER_ASSERT(reporter, ref->getWeakCnt() == 1);
+    SkDEBUGCODE(REPORTER_ASSERT(reporter, ref->getWeakCnt() == 1));
     ref->unref();
 }
 
 DEF_TEST(RefCnt, reporter) {
     test_refCnt(reporter);
     test_weakRefCnt(reporter);
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
@@ skipping 306 matching lines @@
     REPORTER_ASSERT(r, rc->unique());
 
     {
         sk_sp<SkRefCnt> sp = sk_ref_sp(rc);
         REPORTER_ASSERT(r, !rc->unique());
     }
 
     REPORTER_ASSERT(r, rc->unique());
     rc->unref();
 }