Make autofill testable

[emilio]

Filing mostly for tracking, see #27098.

I don't think this one is particularly useful since autofill heuristics are so different (and expected to be different I suspect) across browsers. But for the specific case of testing the effects of autofill on the page like the PR above wants to do it'd come extremely handy :-)

[josepharhar]

If autofill were testable in WPT, then I'd add tests for form controls in ShadowDOM:
WICG/webcomponents#572
whatwg/html#2149

I'm not sure what this would look like though... to test this in chromium, I had to write C++ tests.