Add StringPrintV, fix libxml_utils.cc to use it

base/string_util.h

Index: base/string_util.h
diff --git a/base/string_util.h b/base/string_util.h
index 66518e904333d7966846fbc9f1245b987a08911c..723242e3d09117fd05e4b2b94c57f76351f7aff1 100644
--- a/base/string_util.h
+++ b/base/string_util.h
@@ -453,6 +453,9 @@ double StringToDouble(const string16& value);
 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);