Plug "prompt to unload" into history traversal.

[gterzian]

test: https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/html/browsers/browsing-the-web/unloading-documents/beforeunload-on-navigation-of-parent.html

spec: https://html.spec.whatwg.org/multipage/browsing-the-web.html#prompt-to-unload-a-document

code in Document.prompt_to_unload.

previous discussion at https://html.spec.whatwg.org/multipage/browsing-the-web.html#prompt-to-unload-a-document and #20329 (comment)