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

base/timer.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.
 
 // OneShotTimer and RepeatingTimer provide a simple timer API.  As the names
 // suggest, OneShotTimer calls you back once after a time delay expires.
 // RepeatingTimer on the other hand calls you back periodically with the
 // prescribed time interval.
 //
 // OneShotTimer and RepeatingTimer both cancel the timer when they go out of
@@ skipping 25 matching lines @@
 // allows you to easily defer the timer event until the timer delay passes once
 // again.  So, in the above example, if 0.5 seconds have already passed,
 // calling Reset on timer_ would postpone DoStuff by another 1 second.  In
 // other words, Reset is shorthand for calling Stop and then Start again with
 // the same arguments.
 //
 // NOTE: These APIs are not thread safe. Always call from the same thread.
 
 #ifndef BASE_TIMER_H_
 #define BASE_TIMER_H_
-#pragma once
 
 // IMPORTANT: If you change timer code, make sure that all tests (including
 // disabled ones) from timer_unittests.cc pass locally. Some are disabled
 // because they're flaky on the buildbot, but when you run them locally you
 // should be able to tell the difference.
 
 #include "base/base_export.h"
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
@@ skipping 175 matching lines @@
       : Timer(posted_from, delay,
               base::Bind(method, base::Unretained(receiver)),
               false) {}
 
   void Reset() { Timer::Reset(); }
 };
 
 }  // namespace base
 
 #endif  // BASE_TIMER_H_