This is a followup item from https://chromium-review.googlesource.com/c/chromium/src/+/724724.

Per mfoltz@:

1. Moving the tracking of availability for sets of URLs to PresentationAvailability.cpp; may involve creating an availability object immediately on calling getAvailability().  This would remove some complex code from PresentationAvailabilityState that tracks sets of URLs and improves encapsulation.

2. Seeing if we can use a repeating callback to combine the function of the availability callback and observer (a callback would be provided for each URL in PresentationAvailability).

With these updates, the only data in the state object would be a single map of URL -> list of callback.