figure out better way to give chrome privs to app docs loaded via file: URLs

[mykmelez]

The webview-element branch #60 gives loads in top-level docshells the system principal so Positron can give chrome privileges to application documents that it loads into chrome windows from file: URLs. That works, but it's suboptimal, because it doesn't distinguish between application documents and other documents that load into top-level docshells (like the hidden window). There may also be some security implications.

So we should figure out a better way to give chrome privileges to these documents (and upstream it to Gecko).