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

base/synchronization/spin_wait.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.
 
 // This file provides a macro ONLY for use in testing.
 // DO NOT USE IN PRODUCTION CODE.  There are much better ways to wait.
 
 // This code is very helpful in testing multi-threaded code, without depending
 // on almost any primitives.  This is especially helpful if you are testing
 // those primitive multi-threaded constructs.
 
 // We provide a simple one argument spin wait (for 1 second), and a generic
 // spin wait (for longer periods of time).
 
 #ifndef BASE_SYNCHRONIZATION_SPIN_WAIT_H_
 #define BASE_SYNCHRONIZATION_SPIN_WAIT_H_
-#pragma once
 
 #include "base/threading/platform_thread.h"
 #include "base/time.h"
 
 // Provide a macro that will wait no longer than 1 second for an asynchronous
 // change is the value of an expression.
 // A typical use would be:
 //
 //   SPIN_FOR_1_SECOND_OR_UNTIL_TRUE(0 == f(x));
 //
@@ skipping 14 matching lines @@
       if (kTimeout < base::TimeTicks::Now() - start) { \
       EXPECT_LE((base::TimeTicks::Now() - start).InMilliseconds(), \
                 kTimeout.InMilliseconds()) << "Timed out"; \
         break; \
       } \
       base::PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(50)); \
     } \
   } while (0)
 
 #endif  // BASE_SYNCHRONIZATION_SPIN_WAIT_H_