On the view-source page, generate anchor tags for any 'href' or 'src'
attribute value we come across. This handles both when the attribute
contains an absolute URL and a URL relative to the page.
This requires sending the document's base URL over IPC to resolve
relative URLs.
(cherry picked from commit 1aab7b51ea9c27a6ffff8df0c8bcbec87680865c;
amended to update `on_received_source` signature in Browser too)
52099ff54d