Add ScopedObjCClassSwizzler in base/mac, absorbs objc_method_swizzle and ScopedClassSwizzler

base/test/scoped_class_swizzler_mac.mm

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/test/scoped_class_swizzler_mac.h"
+
+namespace base {
+
+ScopedClassSwizzler::ScopedClassSwizzler(Class target, Class source,
+                                         SEL selector) {
+  old_selector_impl_ = class_getInstanceMethod(target, selector);
+  new_selector_impl_ = class_getInstanceMethod(source, selector);
+  if (!old_selector_impl_ && !new_selector_impl_) {
+    // Try class methods.
+    old_selector_impl_ = class_getClassMethod(target, selector);
+    new_selector_impl_ = class_getClassMethod(source, selector);
+  }
+  method_exchangeImplementations(old_selector_impl_, new_selector_impl_);
+}
+
+ScopedClassSwizzler::~ScopedClassSwizzler() {
+  method_exchangeImplementations(old_selector_impl_, new_selector_impl_);
+}
+
+}  // namespace base