Extract the spinlock from SkOnce as SkSpinlock.

include/core/SkSpinlock.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.
+ */
+
+// This file is not part of the public Skia API.
+
+#ifndef SkSpinlock_DEFINED
+#define SkSpinlock_DEFINED
+
+#include "SkAtomics.h"
+
+#define SK_DECLARE_STATIC_SPINLOCK(name) namespace {} static SkPODSpinlock name
+
+// This class has no constructor and must be zero-initialized (the macro above does this).
+struct SkPODSpinlock {
+    void acquire() {
+        // To act as a mutex, we need an acquire barrier.
+        while(sk_atomic_exchange(&fLocked, true, sk_memory_order_acquire)) { /*spin*/ }
+    }
+    void release() {
+        // To act as a mutex, we need a release barrier.
+        sk_atomic_store(&fLocked, false, sk_memory_order_release);
+    }
+
+    bool fLocked;
+};
+
+// For non-global-static use cases, this is normally what you want.
+class SkSpinlock : public SkPODSpinlock {
+public:
+    SkSpinlock() { this->release(); }
+};
+
+#endif//SkSpinlock_DEFINED