Various clean-ups after top-level lexical declarations are done.

1. Global{Context,Scope}=>Script{Context,Scope}
2. Enable fixed tests
3. Update comments

R=rossberg@chromium.org
BUG=v8:2198
LOG=N

Review URL: https://codereview.chromium.org/716833002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25291 ce2b1a6d-e550-0410-aec6-3dcde31c8c00