Make the strict-mode calling convention for contextual calls the default one.

BUG=
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/131663003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18581 ce2b1a6d-e550-0410-aec6-3dcde31c8c00