Add background fetch WebIDL
<rdar://problem/105213321>
Pull request: https://github.com/WebKit/WebKit/pull/9870
Pull request: https://github.com/apple/WebKit/pull/405
Committed 260532@main (663c322dad1a): <https://commits.webkit.org/260532@main> Reviewed commits have been landed. Closing PR #9870 and removing active labels.