[FCP++] Prevent null from being trace time

We introduced a bug in https://chromium-review.googlesource.com/c/chromium/src/+/1380193.
When image paint detector finds the largest paint candidate with null time, it passes
in to the trace, which will cause a crash.

To fix it, we skip the candidate if the candidate's time is null.

Bug: 920627
Change-Id: Ie39ac9fbc1d09fc88e09851497a80610969bd527
Reviewed-on: https://chromium-review.googlesource.com/c/1407088
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622112}
3 files changed