Extract the spinlock from SkOnce as SkSpinlock.

include/ports/SkAtomics_atomic.h

Index: include/ports/SkAtomics_atomic.h
diff --git a/include/ports/SkAtomics_atomic.h b/include/ports/SkAtomics_atomic.h
index 6ae78a89a97702925597a6be4e9049088a009e6d..ddbf7c3f372124be0881cc4c49308fc5b7f50ad3 100644
--- a/include/ports/SkAtomics_atomic.h
+++ b/include/ports/SkAtomics_atomic.h
@@ -1,3 +1,10 @@
+/*
+ * 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_atomic_DEFINED
 #define SkAtomics_atomic_DEFINED
 
@@ -37,4 +44,10 @@ bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
     return __atomic_compare_exchange_n(ptr, expected, desired, false/*weak?*/, success, failure);
 }
 
+template <typename T>
+T sk_atomic_exchange(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
+    return __atomic_exchange_n(ptr, val, mo);
+}
+
 #endif//SkAtomics_atomic_DEFINED