Previously, the rest api interface set '__isOnParent' for comments.
When comments were added, the property from the comment thread
'isOnParent' was passed as a property. When the draft was saved, it was
expecting '__isOnParent' rather than 'isOnParent' and this caused
comments to show up on the wrong side after saved/refreshed, because
'__isOnParent' was undefined.
Instead of changing to either '__isOnParent' which would be strange to
pass as an attribute or 'isOnParent' which would look like it came from
the api directly, this change introduces isOnParent functions so that
translation doesn't need to be done with the API.
Bug: Issue 5831
Change-Id: I3b849ba5878275cda0a39638626a12bd51341a29