Add debug mirror support for ES6 Map/Set iterators.

This is to show values preview of an iterator in DevTools console.

API=v8::Value::IsMapIterator, v8::Value::IsSetIterator
BUG=chromium:427868
R=arv@chromium.org, yangguo@chromium.org, adamk@chromium.org
LOG=Y

Review URL: https://codereview.chromium.org/693813002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25100 ce2b1a6d-e550-0410-aec6-3dcde31c8c00