Now that there are multiple sources of offline pages:
* Auto-archiving bookmarks
* Last N pages

we need a more coherent strategy for picking the right one to render when a user navigates to a particular online URL.  Doc here:

https://docs.google.com/document/d/1v78bhgRx72tT7KRdfCRSRByOo9Q8KEppjurl5doxoTQ/edit

This will likely happen gradually as we increase the ways to save an offline page, but the basic strategy boils down to:
* If the user explicitly saved a page, use the most recent one
* Otherwise use the most recent automatic page.