[Forward-port to master] - MutationObserver in OverlaySyncService should filter out duplicate callbacks

The current DOM MutationObserver in the OverlaySyncService can easily lead to infinite loops. For example, when a website has a resize callback that mutates the DOM (e.g. set the height of an element) this will trigger the MutationObserver callback, which in turns re-calculates the height of the iframe and subsequently triggering another resize event in the website, starting the infinite loop.

After trying out different solutions (simply checking if the width/height of the site has changed, or fixing the script in the website to only cause a DOM mutation when necessary) it became clear that the list of MutationRecords provided by the MutationObserver is always the same when we are in the loop. This allows for a generic fix that only triggers a redraw in the ChannelManager overlay if the current list of MutationRecords differers from the previous list.

For a more thorough explanation, please see the linked HIPCON-2018 issue.