Add a missing handle scope when clearing the debug message handler.

BUG=none
TEST=cctest/test-debug/DebuggerUnload 
Review URL: http://codereview.chromium.org/126271

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2206 ce2b1a6d-e550-0410-aec6-3dcde31c8c00