Removed the implicit call to DebugBreak when receiving debugger commands

Removed the implicit call to DebugBreak when receiving debugger commands
while V8 is running.

A debugger using the V8 message based interface now needs to control the
call to DebugBreak in order for the messages send to be processed. Commands can
still be send when V8 is not in a break, but they will not be processed until
there is a break. The response "request queued" when queuing up messages have
been removed. This gets rid of a non JSON message being used.

Modified the threaded debugger tests to call DebugBreak instead of relying
on it occouring automatically.

This change will not be committed until the outstanding Chrome change
http://chrome-reviews.prom.corp.google.com/2072 (which updates Chrome to control
the DebugBreak call) have been reviewed and committed.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@47 ce2b1a6d-e550-0410-aec6-3dcde31c8c00