Make HTMLEditor::FocusedElementOrDocumentBecomesNotEditable set `HTMLEditor::mIsInDesignMode
This is a simple mistake, it does not reset mIsInDesignMode when it emulates
a "blur".
Additionally, I realized that it does not check whether the focused element is
in the document or not and emulates "focus" of itself it the focused element is
in a different document. In such case, the editor shouldn't have focus, so,
this is a hidden bug, but "blur" notification may occur asynchronously.
Therefore, this could be a bug only in some edge cases.
Differential Revision: https://phabricator.services.mozilla.com/D216907
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1908239
gecko-commit: f26eadd37a81f8132fa663ceb93d2f542c8bbb58
gecko-reviewers: m_kato