commit 8996d0122ced2d6af980905ac33383e799eff31b
author Liquan(Max) Gu <maxlg@chromium.org> Mon Aug 12 20:18:19 2019
committer Nicolás Peña Moreno <npm@chromium.org> Mon Aug 12 20:18:19 2019
tree 9f54026ac93429258533f84731397ba5404fc5eb
parent 34fd49162ba4ab8ff9224fc5002c5dbdc02e918d

[M77][LCP] Update LCP when both text and image are ready

Largest Contentful Paint compares the text and the image candidate
to decide which is the largest candidate. Currently, LCP generates
a result entry between updating the text candidate and the image
candidate, which produces an intermediate result. It causes
the issue in crbug.com/988115, #c4 explains the mechanism of the bug.

To remove the intermediate state, this CL changes the way
LCP-calculator takes the text candidate and image candidate. Instead
of notifying LCP-calculator of the text candidate's update right after
the update, we wait until both text candidate and image candidate have
been updated to notify LCP-calculator.

Also, LCP-calculator adds the logic of checking whether candidate
has changed, from ImagePaintTimingDetector and TextPaintTimingDetector.
Although it's redundant with |UpdateCandidate| in text and image,
these are necessary because going forwards, the "has_changed" logic in
both detectors would have to be removed along with the LIP and LTP
logic.

This CL also considers the chances where either detector is destroyed,
where it would no longer be able to find the largest candidate. In
this case, perf API would use the last reported candidate as the
candidate to compare with the candidate from another detector, in
order to decide the largest between text and image.

Bug: 982307, 988115

(cherry picked from commit 77c7dc37185a13f5e379b0e8338f0ed3bd0f1f01)

Change-Id: I562c7aeab6037678dd4bd7d6eddacdeb5be8bfbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733456
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#685300}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1749843
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/branch-heads/3865@{#333}
Cr-Branched-From: 0cdcc6158160790658d1f033d3db873603250124-refs/heads/master@{#681094}