Have the icon.size default to computed font size.

This is a proposal to have the icon size default to computed font size. Very often, icons are presented next to text of matching size. This would make it possible to set the font size with CSS and have the icon automatically resize. Does this sound useful?
This method requires that the computed style is correct at the time of `updateIcon`. Also, calling `getComputedStyle` can force a style recalc, so we must use it carefully. To be clear, I'm a big fan of this idea, but I'm concerned it's harder to implement than this.