They and their callees work with the result of `GetRoot()` which is the document
element or the body element. If the body is not editable, `Selection` should
not be updated in non-editable region nor `<br>` elements should not be
inserted in both non-focused editable elements and non-editable elements.
Therefore, they should run only when the document element or the `<body>`
element is editable.
To keep testing crashtests as reported, this patch makes tests which have
`contenteditable` except `<html>` and `<body>` initialize `Selection` as
what we've done. And clean up the tests for helping to port them to WPT
in the future (bug 1725850).
Differential Revision: https://phabricator.services.mozilla.com/D157408
fb15cfbaa6