resource: start storing initialIntersection

[samouri]

summary
Ads passes along the initialIntersection to iframes on creation. We have 3 options:

1. Start providing initialIntersection to layoutCallback
2. Add the field as deprecated to Resource (this is the PRs current state)
3. Delay ads rendering (its all happening in promise chains anyway) by however long it takes for a new one-off InOb measurement

Partial for #31540

[samouri]

I was hoping to not make this API and instead switch to async measures using real InOb.
On further inspection though, it would be a gargantuan task to try and make this async since it is used in so many places that expect it to be sync (For example, any place that calls getIframe)