Move ASCIIToWide and ASCIIToUTF16 to utf_string_conversions.h. I've found it...

base/string_util.h

 // Copyright (c) 2010 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 defines utility functions for working with strings.
 
 #ifndef BASE_STRING_UTIL_H_
 #define BASE_STRING_UTIL_H_
 #pragma once
 
@@ skipping 209 matching lines @@
 bool ContainsOnlyWhitespaceASCII(const std::string& str);
 bool ContainsOnlyWhitespace(const string16& str);
 
 // Returns true if |input| is empty or contains only characters found in
 // |characters|.
 bool ContainsOnlyChars(const std::wstring& input,
                        const std::wstring& characters);
 bool ContainsOnlyChars(const string16& input, const string16& characters);
 bool ContainsOnlyChars(const std::string& input, const std::string& characters);
 
-// These convert between ASCII (7-bit) and Wide/UTF16 strings.
+// Converts to 7-bit ASCII by truncating. The result must be known to be ASCII
+// beforehand.
 std::string WideToASCII(const std::wstring& wide);
-std::wstring ASCIIToWide(const base::StringPiece& ascii);
 std::string UTF16ToASCII(const string16& utf16);
-string16 ASCIIToUTF16(const base::StringPiece& ascii);
+
+// Forward-declares for functions in utf_string_conversions.h. They used to
+// be here and they were moved. We keep these here so the entire project
+// doesn't need to be switched at once.
+// TODO(brettw) delete these when everybody includes utf_string_conversions.h
+// instead.
+std::wstring ASCIIToWide(const char* ascii);
+std::wstring ASCIIToWide(const std::string& ascii);
+string16 ASCIIToUTF16(const char* ascii);
+string16 ASCIIToUTF16(const std::string& ascii);
 
 // Converts the given wide string to the corresponding Latin1. This will fail
 // (return false) if any characters are more than 255.
 bool WideToLatin1(const std::wstring& wide, std::string* latin1);
 
 // Returns true if the specified string matches the criteria. How can a wide
 // string be 8-bit or UTF8? It contains only characters that are < 256 (in the
 // first case) or characters that use only 8-bits and whose 8-bit
 // representation looks like a UTF-8 string (the second case).
 //
@@ skipping 451 matching lines @@
 #elif defined(WCHAR_T_IS_UTF32)
   typedef uint32 Unsigned;
 #endif
 };
 template<>
 struct ToUnsigned<short> {
   typedef unsigned short Unsigned;
 };
 
 #endif  // BASE_STRING_UTIL_H_