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

base/memory/linked_ptr.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.
 //
 // A "smart" pointer type with reference tracking.  Every pointer to a
 // particular object is kept on a circular linked list.  When the last pointer
 // to an object is destroyed or reassigned, the object is deleted.
 //
 // Used properly, this deletes the object when the last reference goes away.
 // There are several caveats:
@@ skipping 18 matching lines @@
 //   A linked_ptr is NOT thread safe. Copying a linked_ptr object is
 //   effectively a read-write operation.
 //
 // Alternative: to linked_ptr is shared_ptr, which
 //  - is also two pointers in size (8 bytes for 32 bit addresses)
 //  - is thread safe for copying and deletion
 //  - supports weak_ptrs
 
 #ifndef BASE_MEMORY_LINKED_PTR_H_
 #define BASE_MEMORY_LINKED_PTR_H_
-#pragma once
 
 #include "base/logging.h"  // for CHECK macros
 
 // This is used internally by all instances of linked_ptr<>.  It needs to be
 // a non-template class because different types of linked_ptr<> can refer to
 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
 // So, it needs to be possible for different types of linked_ptr to participate
 // in the same circular linked list, so we need a single class type here.
 //
 // DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
@@ skipping 123 matching lines @@
 
 // A function to convert T* into linked_ptr<T>
 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
 template <typename T>
 linked_ptr<T> make_linked_ptr(T* ptr) {
   return linked_ptr<T>(ptr);
 }
 
 #endif  // BASE_MEMORY_LINKED_PTR_H_