On macOS with UI side compositing enabled, this is ifdef-ed out, which is the cause of most of the hidpi test failures.
rdar://102277376
Pull request: https://github.com/WebKit/WebKit/pull/6448
Committed 256628@main (285aa4f77035): <https://commits.webkit.org/256628@main> Reviewed commits have been landed. Closing PR #6448 and removing active labels.