Make the HTMLEditor::InsertParagraphSeparatorAsSubAction callers compute the editing host no limited in the `<body
While handling an "insertParagraph" command, it's closed at least in the editing
host. Therefore, it's file to use editing host outside <body> because it
won't make the tree messy outside the editing host.
Differential Revision: https://phabricator.services.mozilla.com/D213376
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1728566
gecko-commit: 8469e6fd08fd0e6865551d5c803bbaa6dd826e85
gecko-reviewers: m_kato