They are currently often wiped and recreated during page loading due to eden collections.
Created attachment 299179 [details] patch
Comment on attachment 299179 [details] patch r=me
Comment on attachment 299179 [details] patch Clearing flags on attachment: 299179 Committed r210891: <http://trac.webkit.org/changeset/210891>
All reviewed patches have been landed. Closing bug.