Make Object::IsFoo const.

Removed a few useless const_casts on the way.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/351573002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21961 ce2b1a6d-e550-0410-aec6-3dcde31c8c00