Skip write barriers in the fast case when setting up local context.

The FastNewContextStub always allocates in new space, so we don't
need to update the write barrier when copying the parameters to
the newly allocated context.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/297203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21481 ce2b1a6d-e550-0410-aec6-3dcde31c8c00