Now mark SetElement as must use result

The debugger runs inside its own context so there should be no setters hit. Which is the reason for the live-edit code asserting non-empty handles.
Review URL: http://codereview.chromium.org/6621042

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7088 ce2b1a6d-e550-0410-aec6-3dcde31c8c00