Add thread information to the debugger.

Add thread information to the debugger.

Each thread running V8 code is assigned an id in thread local storage the first time V8 code is run in it. The thread information returned to the debugger contains the number of threads, the id of each of these threads and which one is the current thread.

Added a threads command to the developer shell debugger for showing information on threads.
Review URL: http://codereview.chromium.org/48009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1523 ce2b1a6d-e550-0410-aec6-3dcde31c8c00