Right now, if the download of the site list fails, we don't show any popular sites (because we don't have any). We should add a fallback list baked into Chrome for those cases (only the list itself, no icons).

Overall, the fallback logic should be:
1. Country-specific list
2. Default list from the server
3. Fallback list baked into Chrome

We also need to make sure that we never fall back to a "lower tier" if something goes wrong. E.g. if we can't reach the server while trying to refresh the list, then we should keep the one we have, rather than falling back to the baked-in list.