[FCP++] Report Largest Content Paint to UKM & UMA

In order to make FCP++ deal with both text and image, we combine Largest Image
Paint and Largest Text Paint into Largest Content Paint.

The largest content paint is the paint time of either the largest text paint or
the largest image paint, depending on whether the largest text or the largest
image is larger. We take whichever larger to be the largest content and use its
paint time as Largest Content Paint.

Bug: 919619
Change-Id: I623e0682b43b2a98ec681b3988159780ac0f7adb
Reviewed-on: https://chromium-review.googlesource.com/c/1399282
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622054}
10 files changed