Include harmony-string-padding.js in the context snapshot

test/mjsunit/harmony/harmony-string-pad-start.js

 // Copyright 2016 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: --harmony-string-padding
-
 (function TestMeta() {
   assertEquals(1, String.prototype.padStart.length);
   assertEquals("function", typeof String.prototype.padStart);
   assertEquals(Object.getPrototypeOf(Function),
                Object.getPrototypeOf(String.prototype.padStart));
   assertEquals("padStart", String.prototype.padStart.name);
 
   var desc = Object.getOwnPropertyDescriptor(String.prototype, "padStart");
   assertFalse(desc.enumerable);
   assertTrue(desc.configurable);
@@ skipping 72 matching lines @@
     var actual = "123".padStart(i, "xoxo");
     assertEquals(expected, actual);
     assertEquals(i > "123".length ? i : 3, actual.length);
   }
 })();
 
 
 (function TestTruncation() {
   assertEquals("ba", "a".padStart(2, "bc"));
 })();