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

test/mjsunit/migrations.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-ayle license that can be
 // found in the LICENSE file.
 
+// Flags: --harmony-object-observe
 // Flags: --allow-natives-syntax --track-fields --expose-gc
 
 var global = Function('return this')();
 var verbose = 0;
 
 function test(ctor_desc, use_desc, migr_desc) {
   var n = 5;
   var objects = [];
   var results = [];
 
@@ skipping 275 matching lines @@
 
 
 
 migrations.forEach(function(migr) {
   uses.forEach(function(use) {
     ctors.forEach(function(ctor) {
       test(ctor, use, migr);
     });
   });
 });