Bug 1020862 - Calculator Visual Refresh follow-up

The second argument to getComputedStyle is optional, so let's leave it out.  Actually, I'm curious why we're using this API at all.  From [MDN](https://developer.mozilla.org/en-US/docs/Web/API/window.getComputedStyle#Description): "The returned object is of the same type that the object returned from the element's style property; however, the two objects have different purposes. The object returned from getComputedStyle is read-only and can be used to inspect the element's style (including those set by a <style> element or an external stylesheet). The elt.style object should be used to set styles on a specific element." 

I'm going to disagree with MDN here.  There's no harm in using `.style` to read and this API is too verbose.  Just access `this.display.fontSize`.