Extract the spinlock from SkOnce as SkSpinlock.

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index cdd39ef4b753ffdba6ccc577eb014f2868451dcd..9866db4860cea31df1289391748e23bb7288cc58 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.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_DEFINED
 #define SkAtomics_DEFINED
 
@@ -27,6 +34,9 @@ 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>