[manifest] Resolve start_url against manifest URL (#330)

Proposal: If manifest URL must be same-origin with document URL and document URL must be same-origin with start_url, then can we simplify the steps for processing a start_url by simply resolving it against manifest URL, without needing to make reference to document URL?

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/330