RemoteScrollingTree has some platform #ifdefs. Clean up by adding subclasses.
<rdar://problem/101405405>
Pull request: https://github.com/WebKit/WebKit/pull/5609
Committed 255843@main (22dc6498963e): <https://commits.webkit.org/255843@main> Reviewed commits have been landed. Closing PR #5609 and removing active labels.