So that when a node is removed and this node is related to the start or
the end point of the `CrossShadowBoundaryRange`, we can clear this
`CrossShadowBoundaryRange`. We clear it for now because we aren't sure
about what the new points should be.
`nsRange` does a similar thing to `mRoot`, and we can't rely on `nsRange` to
observer `mRoot` because `mRoot` could be root of a collapsed range, so
it's not the root of `CrossShadowBoundaryRange`.
Differential Revision: https://phabricator.services.mozilla.com/D211246
aa5e50a334
