Very similar to the current interface, but matches the new C++ structure, and
exposes the stats values as Objects which can be downcast to more specific
types (where the previous API only exposed the values as strings).
BUG=webrtc:6871
Review-Url: https://codereview.webrtc.org/2807933003
Cr-Commit-Position: refs/heads/master@{#17746}
