Add WebProcess initial implementation of background fetch
<rdar://problem/105262330>
Pull request: https://github.com/WebKit/WebKit/pull/9913
Pull request: https://github.com/apple/WebKit/pull/407
Pull request: https://github.com/WebKit/WebKit/pull/10438
Committed 260602@main (20d85cda31cf): <https://commits.webkit.org/260602@main> Reviewed commits have been landed. Closing PR #9913 and removing active labels.