caretPositionFromPoint reveals nodes in shadow dom, if the target is a text widget

[smaug----]

#10200 didn't fix shadow dom handling for step 4 in https://drafts.csswg.org/cssom-view/Overview#dom-document-caretpositionfrompoint, only step 5

Looks like the tests don't match the spec, but I assume the issue is in the spec.

@siliu1