Extract the spinlock from SkOnce as SkSpinlock.

include/ports/SkAtomics_std.h

Index: include/ports/SkAtomics_std.h
diff --git a/include/ports/SkAtomics_std.h b/include/ports/SkAtomics_std.h
index e89d74981c449c228899419591c717fb3077155f..4c26858dfda4b0b22be881b3be7b37b05a9e2d26 100644
--- a/include/ports/SkAtomics_std.h
+++ b/include/ports/SkAtomics_std.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_std_DEFINED
 #define SkAtomics_std_DEFINED
 
@@ -47,4 +54,11 @@ bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
                                                         (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