Fix thread-safety issue in [WKProcessAssertionBackgroundTaskManager _handleBackgroundTaskExpiration].
<rdar://problem/63569049>
Created attachment 400763 [details] Patch
Comment on attachment 400763 [details] Patch r=me
Committed r262399: <https://trac.webkit.org/changeset/262399> All reviewed patches have been landed. Closing bug and clearing flags on attachment 400763 [details].