Extract the spinlock from SkOnce as SkSpinlock.

include/core/SkAtomics.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 #ifndef SkAtomics_DEFINED
 #define SkAtomics_DEFINED
 
 // This file is not part of the public Skia API.
 #include "SkTypes.h"
 
 enum sk_memory_order {
     sk_memory_order_relaxed,
     sk_memory_order_consume,
     sk_memory_order_acquire,
     sk_memory_order_release,
     sk_memory_order_acq_rel,
     sk_memory_order_seq_cst,
 };
 
 template <typename T>
 T sk_atomic_load(const T*, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
 void sk_atomic_store(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
 T sk_atomic_fetch_add(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
 bool sk_atomic_compare_exchange(T*, T* expected, T desired,
                                 sk_memory_order success = sk_memory_order_seq_cst,
                                 sk_memory_order failure = sk_memory_order_seq_cst);
 
+template <typename T>
+T sk_atomic_exchange(T*, T, sk_memory_order = sk_memory_order_seq_cst);
+
 // A little wrapper class for small T (think, builtins: int, float, void*) to
 // ensure they're always used atomically.  This is our stand-in for std::atomic<T>.
 template <typename T>
 class SkAtomic : SkNoncopyable {
 public:
     SkAtomic() {}
 
     // It is essential we return by value rather than by const&.  fVal may change at any time.
     T load(sk_memory_order mo = sk_memory_order_seq_cst) const {
         return sk_atomic_load(&fVal, mo);
@@ skipping 52 matching lines @@
 template <typename T>
 T sk_acquire_load(T* ptr) { return sk_atomic_load(ptr, sk_memory_order_acquire); }
 
 template <typename T>
 void sk_release_store(T* ptr, T val) { sk_atomic_store(ptr, val, sk_memory_order_release); }
 
 inline void sk_membar_acquire__after_atomic_dec() {}
 inline void sk_membar_acquire__after_atomic_conditional_inc() {}
 
 #endif//SkAtomics_DEFINED