The logic for choosing the principal here was originally written before
loadInfo had a separate principalToInherit field, and we needed to specify it
via the triggeringPrincipal instead. At that point, we had to choose a
component principal with permission to load the URI at the start of the
request.
However, now that we have a separate field for the principal to inherit, it's
only needed after access checks have passed and we know that we have a URI
which inherits a principal. In that case, the current logic causes us to
always inherit the first principal in the whitelist (which is the page
principal) for URIs (such as data: URIs) that always inherit, where we really
want to inherit the last (which is the extension principal).
MozReview-Commit-ID: EPoUNuOCwrH
--HG--
extra : rebase_source : 535083e3dd4598ac4e8ea187d27d61ac2ac48476
7cdbab4b1e