Have iOS use the same atomicops_internal implementation that Mac uses.

base/atomicops_internals_mac.h

 // Copyright (c) 2011 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.
 
 // This file is an internal atomic implementation, use base/atomicops.h instead.
 
-#ifndef BASE_ATOMICOPS_INTERNALS_X86_MACOSX_H_
-#define BASE_ATOMICOPS_INTERNALS_X86_MACOSX_H_
+#ifndef BASE_ATOMICOPS_INTERNALS_MAC_H_
+#define BASE_ATOMICOPS_INTERNALS_MAC_H_
 
 #include <libkern/OSAtomic.h>
 
 namespace base {
 namespace subtle {
 
 inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32 *ptr,
                                          Atomic32 old_value,
                                          Atomic32 new_value) {
   Atomic32 prev_value;
@@ skipping 168 matching lines @@
 inline Atomic64 Release_Load(volatile const Atomic64 *ptr) {
   MemoryBarrier();
   return *ptr;
 }
 
 #endif  // defined(__LP64__)
 
 }   // namespace base::subtle
 }   // namespace base
 
-#endif  // BASE_ATOMICOPS_INTERNALS_X86_MACOSX_H_
+#endif  // BASE_ATOMICOPS_INTERNALS_MAC_H_