The problem is that worker manager re-uses complete url with query parameters for spin-off worker front-ends. In case of remote debugging, newly created front-end attempts to connect to the same web socket (because of the ws parameter).
Created attachment 155035 [details]
Created attachment 155042 [details]
Comment on attachment 155042 [details]
Clearing flags on attachment: 155042
Committed r124005: <http://trac.webkit.org/changeset/124005>
All reviewed patches have been landed. Closing bug.