Enqueue a request to prefetch the specified URI.
| aURI | the URI of the document to prefetch |
| aReferrerURI | the URI of the referring page |
| aSource | the DOM node (such as a tag) that requested this fetch, or null if the prefetch was not requested by a DOM node. |
| aExplicit | the link element has an explicit prefetch link type |
Enumerate the items in the prefetch queue.