Fix Context::declaration_context to account for script contexts.

R=rossberg@chromium.org
BUG=v8:3690
LOG=N

Review URL: https://codereview.chromium.org/715263003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25303 ce2b1a6d-e550-0410-aec6-3dcde31c8c00