Make accessors for oddball objects return Oddball* instead of Object*.

Make accessors for oddball objects return Oddball* instead of Object*.

Fix a use of the hole value and the undefined value before initialization when
initializing V8. Before we just read a NULL value from them.
Review URL: http://codereview.chromium.org/8130002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9557 ce2b1a6d-e550-0410-aec6-3dcde31c8c00