The referrer info is more directly what we want (what triggered this
load).
I could add this information for non-http channels, but I think it's
probably not worth it. This is useful because it's thread-safe, which
allows me to support off-main-thread parsing of stylesheets that contain
SRI metadata.
It also simplifies the API and avoids getting URIs around in non-error
code paths.
Differential Revision: https://phabricator.services.mozilla.com/D238612
db9bcd53f7