DevTools: Show preview for ES6 MapIterator, SetIterator in console.

BUG=427868
R=yurys, vsevik

Review URL: https://codereview.chromium.org/694753002

git-svn-id: svn://svn.chromium.org/blink/trunk@185029 bbb929c8-8fbe-4397-9dbb-9b2b20218538