Use a spinlock in SkOnce.

src/ports/SkAtomics_none.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkAtomics_none_DEFINED
 #define SkAtomics_none_DEFINED
 
@@ skipping 20 matching lines @@
 }
 
 static inline void sk_membar_acquire__after_atomic_dec() { }
 
 static inline int32_t sk_atomic_conditional_inc(int32_t* addr) {
     int32_t value = *addr;
     if (value != 0) ++*addr;
     return value;
 }
 
+static inline bool sk_atomic_cas(int32_t* addr, int32_t before, int32_t after) {
+    if (*addr != before) return false;
+    *addr = after;
+    return true;
+}
+
 static inline void sk_membar_acquire__after_atomic_conditional_inc() { }
 
 #endif