Make Fathom see into iframes

[erikrose]

Fathom's input comes from querySelector calls. Those cannot penetrate into iframes of different origins due to the same-origin policy. See if we can get around that somehow, perhaps with privileged code. It would be useful for credit-card autofill.

We should be able to see into iframes of the same origin. Confirm this is the case.

[DimiDL]

To make this issue more clear:

• For cross-origin iframes, we can't see elements inside the cross-origin iframe due to same-origin-policy
• For same-origin iframes, although we can see elements inside an iframe, querySelector doesn't search elements outside the document.

To support penetrate same-origin iframes, the solutions I can think of currently is either extending the querySelector call to include iframes, Ex, querySelector("iframe").contentDocument.querySelector(selector) or performing vectorization in every same-origin document.