Remove references to `window`.

These references are already setup properly using `this` which will work
even in non-browser environments or when we do not want to be working in
globals mode (for example when supplying `Ember.lookup` or
`Ember.exports` as mentioned in https://github.com/emberjs/ember.js/commit/305202f).
Merge pull request #4509 from rjackson/remove-references-to-window

Remove references to `window`.