Ref document in Document::postTask() lambda to make sure it stay alive during the task execution.
Pull request: https://github.com/WebKit/WebKit/pull/9609
Committed 259841@main (f223f5839426): <https://commits.webkit.org/259841@main> Reviewed commits have been landed. Closing PR #9609 and removing active labels.
<rdar://problem/105021459>