harmony-scoping: Implement StoreIC handler for stores to global contexts.

R=ishell@chromium.org, adamk@chromium.org, rossberg@chromium.org, verwaest@chromium.org
BUG=v8:2198
LOG=N

Review URL: https://codereview.chromium.org/712973002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25264 ce2b1a6d-e550-0410-aec6-3dcde31c8c00