ContentPanel subscribes to its own SizeChanged event

### Description

ContentPanel subscribes to its own SizeChanged event

ContentPanel.cs, the backing panel for ContentView on Windows. In the constructor, someone added a subscription to the ContentPanel's _own_ SizeChanged event and handles it to update some border and clipping stuff. Any change in size of the ContentPanel is going to be preceded by a measure/arrange pass; any changes to the border or clipping size/shape should be handled during the arrange step. Handling it after SizeChanged risks kicking off a whole other measure/arrange pass, which is bad for performance. And worse, it might make the border/clipping lag behind the size/shape changes of the panel itself.
I'm not 100% certain this is wrong, but if there is a good reason to do it this way, then it absolutely needs to be called out in the comments, because it's weird.