Zap map value in the stub-cache on clear.
The empty_string is a valid key for keyed loads and stores.

BUG=
R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15565002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14732 ce2b1a6d-e550-0410-aec6-3dcde31c8c00