Use MigrateToMap instead of set_map to update the map of a JSObject.
This is necessary to guarantee correct representation usage.
Some unhandlified code still needs to be handlified before we can push this
through fully.

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/194783002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19844 ce2b1a6d-e550-0410-aec6-3dcde31c8c00