[LargestContentfulPaint] Ship LargestContentfulPaint API

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/S36p6Y16u5c

Bug: 965505, 972038
Change-Id: Id2aac424b508ea698da5c73972d4c1b0442a2618
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715739
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Auto-Submit: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681116}
2 files changed