Fix UnboundScript::GetScriptName and GetLineNumber.

Probably broken since r19925 (mine).

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/296043004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21398 ce2b1a6d-e550-0410-aec6-3dcde31c8c00