Beacon is picking mp4 as an LCP element in video

[DahmaniAdame]

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Describe the bug
In the following structure:

<video loop="loop" autoplay="" playsinline="" muted="" aria-hidden="true" id="" preload="none" src="video-1.mp4">
<source type="video/mp4" src="video-2.mp4">
</video>

The beacon is picking video-2.mp4 as the LCP element.
It's caused by the following fall back (source ? source.src : '') here:

if (nodeName === "video") {
	element_info.type = "img";
	const source = element.querySelector('source');
	element_info.src = element.poster || (source ? source.src : '');
	element_info.current_src = element_info.src;
}

The beacon will initially look for the poster attribute. If not found, and if there is a nested source, it will pick src.

The only exploitable image on a video tag is the poster. And it only works for the video tag for now.

There is no need to look on nested source elements as they won't have a poster, at least until the related proposal comes to life whatwg/html#4004 (comment)

To Reproduce
Same as above.

Expected behavior
Assessing video tags should be limited to the poster attribute.

Screenshots
N/A

Additional context
https://secure.helpscout.net/conversation/2624385201/497095/

Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming