Specs: Revamp how styling works to make it possible
 - for layout manager to animate things,
 - for elements to continue to live in the render tree after they've left the DOM,
 - to define custom units like 'em' for 'vh',
 - to define custom properties like 'font-size'...

(Examples haven't yet been updated.)

Review URL: https://codereview.chromium.org/810173002