Fix date cache in strict mode.

BUG=v8:3220
LOG=N
TEST=mjsunit/regress/regress-3220.js
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/201753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20006 ce2b1a6d-e550-0410-aec6-3dcde31c8c00