Introduce a PrototypeIterator class and use it for prototype access

The new pattern is that we first get the map of the root of the
prototype chain using Object::GetMapRoot() and then walk up the
prototype chain using Map::prototype().

BUG=???
R=verwaest@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/376233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22365 ce2b1a6d-e550-0410-aec6-3dcde31c8c00