Make object accessors more const-correct.

Getting closer to making our IsFOO and FOO::cast methods const-correct...

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/349623002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21897 ce2b1a6d-e550-0410-aec6-3dcde31c8c00