Rebase LCP on top of w3c/paint-timing#100 (#121)

This eliminates the dependency on the still-incubating ElementTiming spec, by using concepts newly-introduced into Paint Timing.

The concept of LCP-eligibility is also added, to encapsulate the various heuristics used to filter out elements which are not the primary content of the page (though which would still be considered "contentful" for purposes of calculating First Contentful Paint time.)

Closes: #86, #113