Limit opportunistically scheduled tasks to foreground pages, and only enable the scheduler if we're not in the middle of page load.
rdar://115404985
Pull request: https://github.com/WebKit/WebKit/pull/17721
Committed 267950@main (7966e09d3699): <https://commits.webkit.org/267950@main> Reviewed commits have been landed. Closing PR #17721 and removing active labels.