Clean up hash creation code to use Handle<Smi> where possible

Also remove apparently-bogus TODO and reorder arguments in
Object::GetOrCreateHash to put Isolate first (as seems to
be the custom).

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/268063005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21246 ce2b1a6d-e550-0410-aec6-3dcde31c8c00