Mock scrollbars on iOS take space (unlike all other scrollbars on that platform), but many layout tests hit the async scrolling path which just hides all scrollbars. Let's try making mock scrollbars have zero width. See also a previous attempt to clean this up in bug 261300.
<rdar://problem/126965107>
Pull request: https://github.com/WebKit/WebKit/pull/27675
Pull request: https://github.com/WebKit/WebKit/pull/29063