Add StringPrintV, fix libxml_utils.cc to use it

base/string_util.cc

Index: base/string_util.cc
diff --git a/base/string_util.cc b/base/string_util.cc
index 9db0f03b5c353d2f1378f3b8ab4a1b91015c715f..b38e182ea676496bd94db8d14c261dde4669aea0 100644
--- a/base/string_util.cc
+++ b/base/string_util.cc
@@ -1245,6 +1245,12 @@ std::wstring StringPrintf(const wchar_t* format, ...) {
   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);