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

base/memory/weak_ptr.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.
 
 // Weak pointers help in cases where you have many objects referring back to a
 // shared object and you wish for the lifetime of the shared object to not be
 // bound to the lifetime of the referrers.  In other words, this is useful when
 // reference counting is not a good fit.
 //
 // Thread-safety notes:
@@ skipping 39 matching lines @@
 // Given the above classes, a consumer may allocate a Controller object, call
 // SpawnWorker several times, and then destroy the Controller object before all
 // of the workers have completed.  Because the Worker class only holds a weak
 // pointer to the Controller, we don't have to worry about the Worker
 // dereferencing the Controller back pointer after the Controller has been
 // destroyed.
 //
 
 #ifndef BASE_MEMORY_WEAK_PTR_H_
 #define BASE_MEMORY_WEAK_PTR_H_
-#pragma once
 
 #include "base/basictypes.h"
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
 #include "base/template_util.h"
 #include "base/threading/thread_checker.h"
 
 namespace base {
 
@@ skipping 251 matching lines @@
 
  private:
   internal::WeakReferenceOwner weak_reference_owner_;
   T* ptr_;
   DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_WEAK_PTR_H_