Bug 1740847 - part 1: Track split point at replacing collapsible white-spaces r=m_kato

It `pointToSplit` should be tracked at replacing text, but I have no idea how
to test this because it replaces the text after the split point.

Differential Revision: https://phabricator.services.mozilla.com/D131036

editor/libeditor/WSRunObject.cpp

@@ -2351,6 +2351,8 @@ WhiteSpaceVisibilityKeeper::MakeSureToKeepVisibleWhiteSpacesVisibleAfterSplit(
       // pointToSplit will be referred bellow so that we need to keep
       // it a valid point.
       AutoEditorDOMPointChildInvalidator forgetChild(pointToSplit);
+      AutoTrackDOMPoint trackSplitPoint(aHTMLEditor.RangeUpdaterRef(),
+                                        &pointToSplit);
       if (atNextCharOfStart.IsStartOfContainer() ||
           atNextCharOfStart.IsPreviousCharASCIISpace()) {
         atNextCharOfStart = textFragmentDataAtSplitPoint