Add a --harmony-object-observe runtime flag (on by default)

test/mjsunit/regress/regress-3315.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.
+//
+// Flags: --harmony-object-observe
 
 var indexZeroCallCount = 0;
 var indexOneCallCount = 0;
 var lengthCallCount = 0;
 var acceptList = {
   get 0() {
     indexZeroCallCount++;
     return 'foo';
   },
   get 1() {
     indexOneCallCount++;
     return 'bar';
   },
   get length() {
     lengthCallCount++;
     return 1;
   }
 };
 
 Object.observe({}, function(){}, acceptList);
 assertEquals(1, lengthCallCount);
 assertEquals(1, indexZeroCallCount);
 assertEquals(0, indexOneCallCount);