Deprecate WebKitCSSMatrix in favor for a new interface for matrices.
We can't get rid of it, there's too much web content using WebKitCSSMatrix (all browsers now support it in some way). But per spec window.WebKitCSSMatrix should be an alias for DOMMatrix. Also see https://github.com/heycam/webidl/issues/362
Looks to be in order given LegacyWindowAlias=WebKitCSSMatrix in DOMMatrix.idl.