Use the origin from the document's URL if the name manifest key cannot be generated when creating a synthetic fall-back manifest

[cvan]

When creating a fall-back synthetic manifest (from issue #3), it's possible that the app name cannot be generated if none of the following exist and contain a non-empty value:

meta[name="apple-mobile-web-app-title"],
meta[name="application-name"],
meta[property="og:site_name"],
meta[property="og:title"],
meta[property="twitter:app_name"],
meta[property="twitter:site_name"],
meta[name="twitter:title"],
title

In this case, generate a name based on the origin (e.g., abc.example.com) of the document URL.