ExtensionConfiguration is just a simple container for extension names
(in a perfect world we would use vector<string> and range-based for
loops), and HandleScopeData was in the totally wrong place. Some
additional cleanup on the way, e.g. using the null pattern behind our
external API.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/139393002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18632 ce2b1a6d-e550-0410-aec6-3dcde31c8c00