A callback on the debugger thread when a message is received. Allows the 
embedding application to wake up the main thread in order to handle the 
message. Useful when the embedding application is idle and sitting in a 
select() call.

Patch by Ryan Dahl <coldredlemur@gmail.com>

Review URL: http://codereview.chromium.org/395013


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3326 ce2b1a6d-e550-0410-aec6-3dcde31c8c00