The D8 debugger frontend now only processes JSON messages. Before the debugger events was passed as objects.

Changed the debugger events JSON generation to use the same serailizer as the responses to requests. This provides correct serialization of mirror objects in debugger events. Renamed ResponsePacket to ProtocolMessage as it is not only used for responses.
Review URL: http://codereview.chromium.org/27202

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1374 ce2b1a6d-e550-0410-aec6-3dcde31c8c00