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This CL allows LCP to use the first input or scroll on the page to
invalidate LCP values from any frame. Doc describing the logic:
https://docs.google.com/document/d/1uOLW-uAZwvBLrH5ST0OgdAJMmqY37MqBJEZRzTBKgp4/edit

Bug: 1050727
Change-Id: Id9ad7c06750af110d6b7f3eec8637a9615f270b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083861Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748550}