Add StringPrintV, fix libxml_utils.cc to use it

base/string_util.cc

 // 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.
 
 #include "base/string_util.h"
 
 #include "build/build_config.h"
 
 #include <ctype.h>
 #include <errno.h>
@@ skipping 1227 matching lines @@
 
 std::wstring StringPrintf(const wchar_t* format, ...) {
   va_list ap;
   va_start(ap, format);
   std::wstring result;
   StringAppendV(&result, format, ap);
   va_end(ap);
   return result;
 }
 
+std::string StringPrintV(const char* format, va_list ap) {
+  std::string result;
+  StringAppendV(&result, format, ap);
+  return result;
+}
+
 const std::string& SStringPrintf(std::string* dst, const char* format, ...) {
   va_list ap;
   va_start(ap, format);
   dst->clear();
   StringAppendV(dst, format, ap);
   va_end(ap);
   return *dst;
 }
 
 const std::wstring& SStringPrintf(std::wstring* dst,
@@ skipping 559 matching lines @@
   // Each input byte creates two output hex characters.
   std::string ret(size * 2, '\0');
 
   for (size_t i = 0; i < size; ++i) {
     char b = reinterpret_cast<const char*>(bytes)[i];
     ret[(i * 2)] = kHexChars[(b >> 4) & 0xf];
     ret[(i * 2) + 1] = kHexChars[b & 0xf];
   }
   return ret;
 }