Add a new GetTlsUniqueChannelBinding method to SSLSocket, and implement nss version.

net/socket/socket_test_util.cc

Index: net/socket/socket_test_util.cc
diff --git a/net/socket/socket_test_util.cc b/net/socket/socket_test_util.cc
index ff66830230ad73c2efb139795a2f191a7812f667..070479577c0701c53ac5e7cf2f75bee0575aca67 100644
--- a/net/socket/socket_test_util.cc
+++ b/net/socket/socket_test_util.cc
@@ -717,6 +717,11 @@ int MockClientSocket::ExportKeyingMaterial(const base::StringPiece& label,
   return OK;
 }
 
+int MockClientSocket::GetTLSUniqueChannelBinding(std::string* out) {
+  out->assign("MOCK_TLSUNIQ");
+  return OK;
+}
+
 ServerBoundCertService* MockClientSocket::GetServerBoundCertService() const {
   NOTREACHED();
   return NULL;