Bug 1557479 [wpt PR 17214] - Implement basic LargestContentfulPaint API, a=testonly

Bug 1557479 [wpt PR 17214] - Implement basic LargestContentfulPaint API, a=testonly

Automatic update from web-platform-tests
Implement basic LargestContentfulPaint API

This CL adds a basic implementation of the LargestContentfulPaint API.
The implementation is done behind a flag, and is missing attribution
information and special handling for images.

Explainer: https://github.com/WICG/LargestContentfulPaint
Intent to Implement:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/WVqgYtyaGJk

Bug: 965505
Change-Id: Ic29604349dc60de2808bdef3df4684e81e1845d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1646634
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670911}

--

wpt-commits: 93ba864d402d941a2670681d535e33723e5296c0
wpt-pr: 17214