ReceiverCheckMode needs to be utilized further.

The parameter wasn't being passed appropriately, and there was an extra
opportunity to use mode RECEIVER_IS_STRING in SubStringStub.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/657313005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24704 ce2b1a6d-e550-0410-aec6-3dcde31c8c00