[whatwg] Adding descent to TextMetrics

[Charles Pritchard]

We've been doing some work with html line boxes; mainly, mixed font 
styles in a text area,
using the HTML Canvas element. While Canvas fillText does have the 
textBaseline attribute,
in order to set the current alignment, it does not provide enough 
metrics through the measureText
function for us to exploit the textBaseline attribute across fonts 
(sizes and styles).

Adding descent, to the measureText object would allow us to do more.
Currently, TextMetrics only returns "width". I'm proposing adding 
"descent" to that list.

-Charles