Previously we were using the document's window - this was both contrary
to spec and causing crashes when the document did not have a window (for
instance the `temp_document` in `HTMLParser::parse_html_fragment`.
This means we no longer crash when navigating between pages on
https://rocketlabcorp.com
We were passing in byte offsets instead of UTF-16 code unit offsets,
which could lead to crashes if the offsets found exceeded the number of
code units in text fragments on the page.
Fixes#4908.
Co-authored-by: Tim Ledbetter <tim.ledbetter@ladybird.org>