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

base/memory/singleton_objc.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.
 
 // Support for using the Singleton<T> pattern with Objective-C objects.  A
 // SingletonObjC is the same as a Singleton, except the default traits are
 // appropriate for Objective-C objects.  A typical Objective-C object of type
 // NSExampleType can be maintained as a singleton and accessed with:
 //
 //   NSExampleType* exampleSingleton = SingletonObjC<NSExampleType>::get();
@@ skipping 11 matching lines @@
 //   struct FooSingletonTraits : public DefaultSingletonObjCTraits<Foo> {
 //     static Foo* New() {
 //       return [[Foo alloc] initWithName:@"selecty"];
 //     }
 //   };
 //   ...
 //   Foo* widgetSingleton = SingletonObjC<Foo, FooSingletonTraits>::get();
 
 #ifndef BASE_MEMORY_SINGLETON_OBJC_H_
 #define BASE_MEMORY_SINGLETON_OBJC_H_
-#pragma once
 
 #import <Foundation/Foundation.h>
 #include "base/memory/singleton.h"
 
 // Singleton traits usable to manage traditional Objective-C objects, which
 // are instantiated by sending |alloc| and |init| messages, and are deallocated
 // in a memory-managed environment when their retain counts drop to 0 by
 // sending |release| messages.
 template<typename Type>
 struct DefaultSingletonObjCTraits : public DefaultSingletonTraits<Type> {
   static Type* New() {
     return [[Type alloc] init];
   }
 
   static void Delete(Type* object) {
     [object release];
   }
 };
 
 // Exactly like Singleton, but without the DefaultSingletonObjCTraits as the
 // default trait class.  This makes it straightforward for Objective-C++ code
 // to hold Objective-C objects as singletons.
 template<typename Type,
          typename Traits = DefaultSingletonObjCTraits<Type>,
          typename DifferentiatingType = Type>
 class SingletonObjC : public Singleton<Type, Traits, DifferentiatingType> {
 };
 
 #endif  // BASE_MEMORY_SINGLETON_OBJC_H_