The problem is that WebCore::AccessibilityObject::lineForPosition returns -1, but WebAccessibilityObject::lineBreaks is assuming it's a number >= 0. The crash happens when it tries to allocate a WebVector of size -1.
Created attachment 113666 [details] Patch
Comment on attachment 113666 [details] Patch Can haz layout test?
Created attachment 113721 [details] Patch
Comment on attachment 113721 [details] Patch pretty.
Created attachment 114362 [details] Patch for landing
Comment on attachment 114362 [details] Patch for landing Clearing flags on attachment: 114362 Committed r99770: <http://trac.webkit.org/changeset/99770>
All reviewed patches have been landed. Closing bug.