Adjust window test to not assumed unmaximized

[bbondy]

Steps to reproduce:

1. Open up browser console with Shift+F8
2. chrome.remote.getCurrentWindow().setSize(122, 122)
   And you'll see it doesn't resize.

Or alternative steps to reproduce:
npm run test -- --grep="offsets from the focused window"
And you'll see it fails, you'll have to remove the this.skip from the test first though.

Note this works in .12.15 but you have to use this code:
require('electron').remote.getCurrentWindow().setSize(122, 122)

Once fixed please re-enable the test.

/cc @posix4e, would you mind taking a look since this is introduced in .13?

[bbondy]

It was tested on Ubuntu 16.04.1 LTS

[bbondy]

This turned out not to be only a problem in the tests. Ubuntu maximizes windows by default when they run so the resize wouldn't work in a maximized state. The fix is just to unmaximize in tests before doing the resize.