Iframes aren't reactivated when their page is restored from bfcache #13096

When a traversal restores a bfcached top-level document, the documents of its child navigables aren't reactivated. Get all navigables whose current session history entry will change or reload only checks a navigable's children when its own document doesn't change:

If targetEntry's document is navigable's active document, and targetEntry's document state's reload pending is false, then extend navigablesToCheck with the child navigables of navigable.

The algorithm that gets navigables needing only a history length/index update has the same condition, and reactivate a document doesn't recurse into descendants. So the iframes don't get pageshow, their history.length/index and navigation.entries() aren't updated, and no dispose fires for entries pruned while the page was in bfcache.

entries-after-bfcache-in-iframe.html expects the iframe to get pageshow and dispose and its entries() to shrink. It passes in Chrome Canary and Safari TP, and fails in Firefox Nightly.

Maybe reactivating a document should also reactivate its descendants, each with its own entries for the navigation API.

(Found while looking into #13085.)