Changed the debugger API to allow only one debug event listener to be registered. The public API now only has SetDebugEventListener instead of AddDebugEventListener and RemoveDebugEventListener.

Moved the registrered debug event listener from the context to a global handle in the Debugger class. Storing it in the context did not make much sense.

Changed a lot of tests to handle the API change.

BUG=1242707
Review URL: http://codereview.chromium.org/19753

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1212 ce2b1a6d-e550-0410-aec6-3dcde31c8c00