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

base/string16.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.
 
 #ifndef BASE_STRING16_H_
 #define BASE_STRING16_H_
-#pragma once
 
 // WHAT:
 // A version of std::basic_string that provides 2-byte characters even when
 // wchar_t is not implemented as a 2-byte type. You can access this class as
 // string16. We also define char16, which string16 is based upon.
 //
 // WHY:
 // On Windows, wchar_t is 2 bytes, and it can conveniently handle UTF-16/UCS-2
 // data. Plenty of existing code operates on strings encoded as UTF-16.
 //
@@ skipping 155 matching lines @@
 BASE_EXPORT extern std::ostream& operator<<(std::ostream& out,
                                             const string16& str);
 
 // This is required by googletest to print a readable output on test failures.
 BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
 }
 
 #endif  // WCHAR_T_IS_UTF32
 
 #endif  // BASE_STRING16_H_