Move CSSOM classes to using MutableStylePropertySet over StylePropertySet.
Created attachment 197987 [details] Patch
Created attachment 197990 [details] Patch
Comment on attachment 197990 [details] Patch Attachment 197990 [details] did not pass gtk-ews (gtk): Output: http://webkit-queues.appspot.com/results/146192
Committed r148396: <http://trac.webkit.org/changeset/148396>