[iOS] try using clipToBounds instead of masks for rectangular clips.

[jonahwilliams]

From #142813 , it seems like ClipRects implemented via mask views might create their masks via software rasterization. We might be able to perform rectangular clips more efficiently by using a parent UIViews frame and clipToBoundsProperty.

This is discussed as one of the answers in https://stackoverflow.com/questions/11391058/simply-mask-a-uiview-with-a-rectangle . We could confirm this is working via CPU profiles.

[jmagman]

I'll look at this one.
cc @hellohuanlin