This attribute is not used in Gecko, but exists for use by other
applications. Specifically, the APP_TYPE_EDITOR type is given permission
to load privileged images as tested by browser_docshell_type_editor.js.
Before these changes, that test passed because the docshell was loaded
in a different process, so the cache was empty when each load occurred,
but after my changes the process ends up being re-used, so the image
cache bypasses this check.
This changes the image cache key to also include the app type
information so that it will be compared before re-using the entry.
Differential Revision: https://phabricator.services.mozilla.com/D126557
