Make HTMLEditor::ComputeEditingHostInternal() stop referring mIsInDesignMode and refer focused element in the window
The method may be called without focus. Therefore, it shouldn't refer
mIsInDesignMode and it should refer focused element in the window (including
shadows) if there is no selection ranges.
Differential Revision: https://phabricator.services.mozilla.com/D218726
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1910800
gecko-commit: e837df1a96c6e231b77bd893d8b1149fc1ae4699
gecko-reviewers: m_kato