When focused element becomes editable by `contentedtiable=true`, IME
content observer isn't created on some situations. Then IME focus isn't
set.
At first, when focused element becomes non-editable,
`FocusedElementOrDocumentBecomesNotEditable` will destroy IME content
observer, but editor might not be destroyed completely since HTML content
has multiple `contenteditable` and has focus.
Then, when focused element becomes editable again,
`FocusedElementOrDocumentBecomesEditable` will check selection ancestor
limit, but since editor isn't destroyed by previous `contenteditable`
change, we already have this limit. Then we don't create IME content
observer.
So we should set current IME state and create IME content observer when
becoming editable.
Differential Revision: https://phabricator.services.mozilla.com/D202409
2fddc3b3c8