[Segment Cache] Cancel prefetch on viewport exit (#74671)
We use an IntersectionObserver to prefetch links when they enter the
viewport. This updates the behavior to cancel the prefetch if the link
exits the viewport before it completes.
This can greatly reduce the amount of data transfer caused by
prefetching, however, the impact of this change will depend on the
user's network conditions. The faster the network conditions, the more
likely the link will have already been prefetched by the time the link
exits the screen.
We'll need a different strategy for limiting prefetch data transfer in
fast network conditions, perhaps by tracking and throttling the overall
bitrate.
