Use a spinlock in SkOnce.

src/ports/SkAtomics_android.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_android_DEFINED
 #define SkAtomics_android_DEFINED
 
@@ skipping 18 matching lines @@
         int32_t value = *addr;
         if (value == 0) {
             return 0;
         }
         if (0 == android_atomic_release_cas(value, value + 1, addr)) {
             return value;
         }
     }
 }
 
+static inline __attribute___((always_inline)) bool sk_atomic_cas(int32_t* addr,
+                                                                 int32_t before,
+                                                                 int32_t after) {
+    // android_atomic_release_cas returns 0 for success (if *addr == before and it wrote after).
+    return android_atomic_release_cas(before, after, addr) == 0;
+}
+
 static inline __attribute__((always_inline)) void sk_membar_acquire__after_atomic_conditional_inc() {
     //HACK: Android is actually using full memory barriers.
     //      Should this change, uncomment below.
     //int dummy;
     //android_atomic_acquire_store(0, &dummy);
 }
 
 #endif