Twin of https://codereview.chromium.org/210783003/ and https://codereview.chromium.org/211153003/.

Cleaned up DoChange a bit on the way, making things more uniform across platforms, removed useless comments etc.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/214613004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20376 ce2b1a6d-e550-0410-aec6-3dcde31c8c00