break location [condition]
  clear <breakpoint #>
  backtrace [from frame #] [to frame #]]
  frame <frame #>
  step [in | next | out| min [step count]]
  print <expression>
  source [from line [num lines]]
  scripts
  continue
  help

It is enabled through the option --debugger which is on by default.
Review URL: http://codereview.chromium.org/14509

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@996 ce2b1a6d-e550-0410-aec6-3dcde31c8c00