Consistently use "use strict" where possible.

src/harmony-string.js

 // Copyright 2014 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.
 
-'use strict';
+"use strict";
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $String = global.String;
 // var $Array = global.Array;
 
 // -------------------------------------------------------------------
 
 // ES6 draft 01-20-14, section 21.1.3.13
 function StringRepeat(count) {
@@ skipping 169 matching lines @@
   InstallFunctions($String.prototype, DONT_ENUM, $Array(
     "codePointAt", StringCodePointAt,
     "includes", StringIncludes,
     "endsWith", StringEndsWith,
     "repeat", StringRepeat,
     "startsWith", StringStartsWith
   ));
 }
 
 ExtendStringPrototype();