When message handler is set to NULL and there is no debugger listener the debugger is unloaded immediately unless it's entered, in which case it will be unloaded when last instance of EnterDebugger is destroyed.

Without the change the debugger may crash as Debugger::EventActive(v8::Break) called from OnDebugBreak may clear current debugger context. 

Also when compilation cache was enabled debugger could fail on second attach for the same reason(see AfterCompileMessageWhenMessageHandlerIsReset).

BUG=12404
Review URL: http://codereview.chromium.org/115709

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2035 ce2b1a6d-e550-0410-aec6-3dcde31c8c00