Use StringPiece for UTF string conversions.

net/cert/x509_cert_types_mac.cc

Index: net/cert/x509_cert_types_mac.cc
diff --git a/net/cert/x509_cert_types_mac.cc b/net/cert/x509_cert_types_mac.cc
index 6439c7f3e77391df933b614dd9edda871f1249a2..9dee2b3fb80d02b217a7ba7fa016625f84368146 100644
--- a/net/cert/x509_cert_types_mac.cc
+++ b/net/cert/x509_cert_types_mac.cc
@@ -125,7 +125,7 @@ bool UTF16BigEndianToUTF8(base::char16* chars, size_t length,
                           std::string* out_string) {
   for (size_t i = 0; i < length; i++)
     chars[i] = EndianU16_BtoN(chars[i]);
-  return UTF16ToUTF8(chars, length, out_string);
+  return UTF16ToUTF8(base::StringPiece16(chars, length), out_string);
 }
 
 // Converts big-endian UTF-32 to UTF-8 in a std::string.