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

base/lazy_instance.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.
 
 // The LazyInstance<Type, Traits> class manages a single instance of Type,
 // which will be lazily created on the first time it's accessed.  This class is
 // useful for places you would normally use a function-level static, but you
 // need to have guaranteed thread-safety.  The Type constructor will only ever
 // be called once, even if two threads are racing to create the object.  Get()
 // and Pointer() will always return the same, completely initialized instance.
@@ skipping 16 matching lines @@
 //   static LazyInstance<MyClass> my_instance = LAZY_INSTANCE_INITIALIZER;
 //   void SomeMethod() {
 //     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
 //
 //     MyClass* ptr = my_instance.Pointer();
 //     ptr->DoDoDo();  // MyClass::DoDoDo
 //   }
 
 #ifndef BASE_LAZY_INSTANCE_H_
 #define BASE_LAZY_INSTANCE_H_
-#pragma once
 
 #include <new>  // For placement new.
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/logging.h"
 #include "base/memory/aligned_memory.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/thread_restrictions.h"
@@ skipping 154 matching lines @@
     LazyInstance<Type, Traits>* me =
         reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
     Traits::Delete(me->instance());
     subtle::NoBarrier_Store(&me->private_instance_, 0);
   }
 };
 
 }  // namespace base
 
 #endif  // BASE_LAZY_INSTANCE_H_