in case script name is missing.

BUG=http://crbug.com/39290

Author: Andrey Kosyakov (caseq@chromium.org)
Original issue: http://codereview.chromium.org/1303003

TBR=sgjesse@chromium.org

Review URL: http://codereview.chromium.org/1527007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4337 ce2b1a6d-e550-0410-aec6-3dcde31c8c00