Add RemoteLayerTreeDrawingAreaProxyIOS and move some #ifdeffed code out of RemoteLayerTreeDrawingAreaProxy into it.
<rdar://problem/101109019>
Pull request: https://github.com/WebKit/WebKit/pull/5315
Committed 255484@main (8cf25e21bf55): <https://commits.webkit.org/255484@main> Reviewed commits have been landed. Closing PR #5315 and removing active labels.