Collect the style rules that apply to an inspected element and expose
them through the existing DOM node inspection path. This gives Firefox's
Rules panel real rule forms instead of the previous empty getApplied
response.
When we inspect a DOM node, we currently serialize many properties for
that node, including its layout, computed style, used fonts, etc. Now
that we aren't piggy-backing on the Inspector interface, we can instead
only serialize the specific information required by DevTools.