support environments with both window and global

Such environments include [node-webkit](https://github.com/rogerwang/node-webkit) and [atom-shell](https://github.com/atom/atom-shell).

This clears #1340 

---

~~This breaks the following tests within test/unit/node/generated-code-dependencies.js:~~
* ~~`test('class'`~~
* ~~`test('generator'`~~
* ~~`test('generator (symbols)'`~~
* ~~`test('compiled modules'`~~
* ~~`test('compiled modules inline'`~~
* ~~`test('compiled modules mix inline & script'`~~

~~I have not yet investigated the `child_process.exec()` based tests, however the `vm.runInNewContext()` tests have an issue with `global` within indirect `eval()`s. It gets pretty deep and difficult to debug. I am still working on this, but any help would be appreciated.~~
It seems to work fine in node-webkit and Safari. In my prior tests with `this` in a Worker, it too worked but as stated above, `this !== self`. If we pass `this`, how will `getGlobal()` differentiate `self`?
```js
(function(thisObject) {
    if (typeof self !== 'undefined') thisObject = self;    // I guess?
})(this);