Bug 1820449 - Make `WhiteSpaceVisibilityKeeper` verify tracking point of start...

Bug 1820449 - Make `WhiteSpaceVisibilityKeeper` verify tracking point of start of right joining node after flushing the tracker r=m_kato

In the legacy mode, right node is not removed from the DOM tree, therefore,
this bug was hidden.  However, after enabling the new mode, the point will
be out of the document.  Therefore, the check will cause
`NS_ERROR_EDITOR_UNEXPECTED_DOM_TREE` error and the joining code stops handling
the deletion.

Differential Revision: https://phabricator.services.mozilla.com/D171822