Because memory usage!
Created attachment 123562 [details]
Quite a bit of code for something that looks like a rather narrow detail. I wonder if some refactoring would eliminate the concept completely. r=me in any case as this is an improvement.
Comment on attachment 123562 [details]
Clearing flags on attachment: 123562
Committed r105631: <http://trac.webkit.org/changeset/105631>
All reviewed patches have been landed. Closing bug.