Move context retrieval method around. Use delegation for implementation.

This is a refactoring of https://code.google.com/p/v8/source/detail?r=14146.

Review URL: https://codereview.chromium.org/13940003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14171 ce2b1a6d-e550-0410-aec6-3dcde31c8c00