Take foreground assertion for client navigation in background views on macOS, like we already do on iOS. This is important now that we've adopted RunningBoard on macOS and that RunningBoard assertion control the priority of our processes.
<rdar://107383904>
Pull request: https://github.com/WebKit/WebKit/pull/15217
Committed 265442@main (985a124eb978): <https://commits.webkit.org/265442@main> Reviewed commits have been landed. Closing PR #15217 and removing active labels.