Remove #pragma once. Just from base/ for now.

base/atomicops.h

 // Copyright (c) 2012 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.
 
 // For atomic operations on reference counts, see atomic_refcount.h.
 // For atomic operations on sequence numbers, see atomic_sequence_num.h.
 
 // The routines exported by this module are subtle.  If you use them, even if
 // you get the code right, it will depend on careful reasoning about atomicity
 // and memory ordering; it will be less readable, and harder to maintain.  If
 // you plan to use these routines, you should have a good reason, such as solid
 // evidence that performance would otherwise suffer, or there being no
 // alternative.  You should assume only properties explicitly guaranteed by the
 // specifications in this file.  You are almost certainly _not_ writing code
 // just for the x86; if you assume x86 semantics, x86 hardware bugs and
 // implementations on other archtectures will cause your code to break.  If you
 // do not know what you are doing, avoid these routines, and use a Mutex.
 //
 // It is incorrect to make direct assignments to/from an atomic variable.
 // You should use one of the Load or Store routines.  The NoBarrier
 // versions are provided when no barriers are needed:
 //   NoBarrier_Store()
 //   NoBarrier_Load()
 // Although there are currently no compiler enforcement, you are encouraged
 // to use these.
 //
 
 #ifndef BASE_ATOMICOPS_H_
 #define BASE_ATOMICOPS_H_
-#pragma once
 
 #include "base/basictypes.h"
 #include "build/build_config.h"
 
 namespace base {
 namespace subtle {
 
 typedef int32 Atomic32;
 #ifdef ARCH_CPU_64_BITS
 // We need to be able to go between Atomic64 and AtomicWord implicitly.  This
@@ skipping 102 matching lines @@
 #error "Atomic operations are not supported on your platform"
 #endif
 
 // On some platforms we need additional declarations to make
 // AtomicWord compatible with our other Atomic* types.
 #if defined(OS_MACOSX) || defined(OS_OPENBSD)
 #include "base/atomicops_internals_atomicword_compat.h"
 #endif
 
 #endif  // BASE_ATOMICOPS_H_