GraphicsLayerCA::constrainedSize and PlatformCALayerWinInternal::constrainedSize both fulfill the same function: to ensure that a tiled layer does not exceed a certain size. We should unify these two functions.
GraphicsLayerCA::constrainedSize handles overflow more nicely. PlatformCALayerWinInternal::constrainedSize does a nicer job of actually constraining. We should take the best parts of both functions.