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

base/memory/singleton.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.
 
 // PLEASE READ: Do you really need a singleton?
 //
 // Singletons make it hard to determine the lifetime of an object, which can
 // lead to buggy code and spurious crashes.
 //
 // Instead of adding another singleton into the mix, try to identify either:
 //   a) An existing singleton that can manage your object's lifetime
 //   b) Locations where you can deterministically create the object and pass
 //      into other objects
 //
 // If you absolutely need a singleton, please keep them as trivial as possible
 // and ideally a leaf dependency. Singletons get problematic when they attempt
 // to do too much in their destructor or have circular dependencies.
 
 #ifndef BASE_MEMORY_SINGLETON_H_
 #define BASE_MEMORY_SINGLETON_H_
-#pragma once
 
 #include "base/at_exit.h"
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/memory/aligned_memory.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/thread_restrictions.h"
 
 namespace base {
 namespace internal {
@@ skipping 244 matching lines @@
     instance_ = 0;
   }
   static base::subtle::AtomicWord instance_;
 };
 
 template <typename Type, typename Traits, typename DifferentiatingType>
 base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
     instance_ = 0;
 
 #endif  // BASE_MEMORY_SINGLETON_H_