add a default value for return value

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/16642003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15024 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
build fix for 15023

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/16729002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15025 ce2b1a6d-e550-0410-aec6-3dcde31c8c00