Rename new_space_dominator to dominator since dominators can also be in old space.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/312713002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21629 ce2b1a6d-e550-0410-aec6-3dcde31c8c00