Fix tests regress-336820 and regress-678917.

test/mjsunit/regress/regress-678917.js

 // Copyright 2017 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// Flags: --max-old-space-size=400
+
 s1 = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
 s1 += s1;
 s1 += s1;
 s1 += s1;
 s1 += s1;
 
 s0 = 'a';
 
 function g() {
   for (var j = 0; j < 1000000; j++) {
     s0 += s1;
   }
 }
 
 try {
   g();
 } catch (e) {
 }
 
 assertEquals('x', s0[10]);