Adjust font size using Control key with Mouse Scroll Wheel.

Following from previous [PR](https://github.com/atom/atom/pull/3475), to allow merging.

This Pull Request adjusts the font size when the control key is held down and scroll wheel is used.

It ignores OSX(darwin) as this platform has a conflicting accessibility feature which should not be overwritten.


Would you mind moving this to the top of this function, under line 592, and returning early  after `event.preventDefault()` is called?

That way it minimizes the diff even further.