Add support for Element Timing

[Jash Sayani]

Add support for Element Timing API: https://wicg.github.io/element-timing/

[Brent Fulgham]

As discussed in webkit-dev mailing list:

"Apple's WebKit team reviewed this API and we have a few concerns including
but not limited to:

   - The proposed API exposes timing at which a given element is painted.
   Implemented naively, this exposes the implementation detail of what kind of
   compositing tiles are used on a given web page. Hiding this
   implementation detail and recording the exact theoretical paint timing will
   be prohibitively expensive to do on all websites.
   - The definition of the set of owned text nodes and how they compute
   intersectionRect seems inadequate. It's unclear what "border box" of
   *Text* node would mean. The spec doesn't seem to ever populate "set of
   elements with rendered text" either.
   - The use of this API seems to incur a significant runtime as well as
   memory cost."

Moving to "RESOLVED | WONTFIX". We can reopen if we revisit this, or the spec issues are resolved.