Add Sk version of std::exchange

src/core/SkExchange.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkExchange_DEFINED
+#define SkExchange_DEFINED
+
+#include <utility>
+
+namespace skstd {
+
+// std::exchange is in C++14
+template<typename T, typename U = T>
+inline static T exchange(T& obj, U&& new_val) {
+    T old_val = std::move(obj);
+    obj = std::forward<U>(new_val);
+    return old_val;
+}
+
+}
+
+#endif  // SkExchange_DEFINED