The issue seems to be that adjustments are attaching to non-MainFrameScrollbarGtk instances.
Created attachment 66727 [details] Fix for the crash
Comment on attachment 66727 [details] Fix for the crash Yay.
Committed r66885: <http://trac.webkit.org/changeset/66885>
Committed r66889: <http://trac.webkit.org/changeset/66889>