Moves the calls to tracer and UpdateLiveObjectCount inside the call to
(inlined) SetMark.  Removes global object counter.
Review URL: http://codereview.chromium.org/8910

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@688 ce2b1a6d-e550-0410-aec6-3dcde31c8c00