Don't attempt to forward declare StringPiece.

base/sys_string_conversions.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.
 
 #ifndef BASE_SYS_STRING_CONVERSIONS_H_
 #define BASE_SYS_STRING_CONVERSIONS_H_
 #pragma once
 
 // Provides system-dependent string type conversions for cases where it's
 // necessary to not use ICU. Generally, you should not need this in Chrome,
 // but it is used in some shared code. Dependencies should be minimal.
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/string16.h"
+#include "base/string_piece.h"
 
 #if defined(OS_MACOSX)
 #include <CoreFoundation/CoreFoundation.h>
 #ifdef __OBJC__
 @class NSString;
 #else
 class NSString;
 #endif
 #endif  // OS_MACOSX
 
 namespace base {
 
-class StringPiece;
-
 // Converts between wide and UTF-8 representations of a string. On error, the
 // result is system-dependent.
 BASE_EXPORT std::string SysWideToUTF8(const std::wstring& wide);
 BASE_EXPORT std::wstring SysUTF8ToWide(const StringPiece& utf8);
 
 // Converts between wide and the system multi-byte representations of a string.
 // DANGER: This will lose information and can change (on Windows, this can
 // change between reboots).
 BASE_EXPORT std::string SysWideToNativeMB(const std::wstring& wide);
 BASE_EXPORT std::wstring SysNativeMBToWide(const StringPiece& native_mb);
@@ skipping 38 matching lines @@
 // string type of length 0.
 BASE_EXPORT std::string SysNSStringToUTF8(NSString* ref);
 BASE_EXPORT string16 SysNSStringToUTF16(NSString* ref);
 BASE_EXPORT std::wstring SysNSStringToWide(NSString* ref);
 
 #endif  // defined(OS_MACOSX)
 
 }  // namespace base
 
 #endif  // BASE_SYS_STRING_CONVERSIONS_H_