All preemption requests are now ignored while in the debugger. This ensures that no change of V8 thread happenes  while in the debugger. The only thing that happens is that a flag is set to indicate that preemption happened. When the debugger is left preemption is requested if it occourred while in the debugger.

Moved the debugger related global variables from Top to thread local in Debug.
Review URL: http://codereview.chromium.org/39124

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1436 ce2b1a6d-e550-0410-aec6-3dcde31c8c00