Resolve an inconsistency between Chrome and Firefox with how the
contentscript runs in an iframe.
This should have no user-facing impact, it's just meant as a safeguard
in case something unintentionally gets included in the contentscript.
The web accessible resources were removed in #14834, but they were
accidentally preserved in the v3 manifest. We no longer need any web
accessible resources, and leaving these in the manifest exposes us to
potential security issues.
