Extract the spinlock from SkOnce as SkSpinlock.

include/ports/SkAtomics_std.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_std_DEFINED
 #define SkAtomics_std_DEFINED
 
 // We try not to depend on the C++ standard library,
 // but these uses of <atomic> should all inline, so we don't feel to bad here.
 #include <atomic>
 
 template <typename T>
 T sk_atomic_load(const T* ptr, sk_memory_order mo) {
     SkASSERT(mo == sk_memory_order_relaxed ||
@@ skipping 29 matching lines @@
              failure == sk_memory_order_seq_cst ||
              failure == sk_memory_order_acquire ||
              failure == sk_memory_order_consume);
     SkASSERT(failure <= success);
     std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
     return std::atomic_compare_exchange_strong_explicit(ap, expected, desired,
                                                         (std::memory_order)success,
                                                         (std::memory_order)failure);
 }
 
+template <typename T>
+T sk_atomic_exchange(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
+    std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
+    return std::atomic_exchange_explicit(ap, val, (std::memory_order)mo);
+}
+
 #endif//SkAtomics_std_DEFINED