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

test/mjsunit/es7/object-observe-runtime.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
 // Flags: --allow-natives-syntax
 
 // These tests are meant to ensure that that the Object.observe runtime
 // functions are hardened.
 
 var obj = {};
 %SetIsObserved(obj);
 assertThrows(function() {
   %SetIsObserved(obj);
 });
 
 assertThrows(function() {
   %SetIsObserved(this);
 });