Add StringPrintV, fix libxml_utils.cc to use it

base/string_util.h

 // Copyright (c) 2006-2008 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_
 
 #include <stdarg.h>   // va_list
@@ skipping 435 matching lines @@
 int64 StringToInt64(const string16& value);
 int HexStringToInt(const std::string& value);
 int HexStringToInt(const string16& value);
 double StringToDouble(const std::string& value);
 double StringToDouble(const string16& value);
 
 // Return a C++ string given printf-like input.
 std::string StringPrintf(const char* format, ...) PRINTF_FORMAT(1, 2);
 std::wstring StringPrintf(const wchar_t* format, ...) WPRINTF_FORMAT(1, 2);
 
+// Return a C++ string given vprintf-like input.
+std::string StringPrintV(const char* format, va_list ap) PRINTF_FORMAT(1, 0);
+
 // Store result into a supplied string and return it
 const std::string& SStringPrintf(std::string* dst, const char* format, ...)
     PRINTF_FORMAT(2, 3);
 const std::wstring& SStringPrintf(std::wstring* dst,
                                   const wchar_t* format, ...)
     WPRINTF_FORMAT(2, 3);
 
 // Append result to a supplied string
 void StringAppendF(std::string* dst, const char* format, ...)
     PRINTF_FORMAT(2, 3);
@@ skipping 161 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_