Add GetSctpStats to PeerConnectionInternal, remove sctp_data_channels()

This removes code from DataChannelController that exposes
an internal vector of data channels and puts the onus of
returning stats for a data channel, on the data channel
object itself. This will come in handy as we make threading
changes to the data channel object.

Change-Id: Ie164cc5823cd5f9782fc5c9a63aa4c76b8229639
Bug: webrtc:11547, webrtc:11687
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177244
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31533}

pc/peer_connection_internal.h

diff --git a/pc/peer_connection_internal.h b/pc/peer_connection_internal.h
index 52ffe85..66d585b 100644
--- a/pc/peer_connection_internal.h
+++ b/pc/peer_connection_internal.h
@@ -46,8 +46,11 @@
   // Only valid when using deprecated RTP data channels.
   virtual cricket::RtpDataChannel* rtp_data_channel() const = 0;
 
-  virtual std::vector<rtc::scoped_refptr<DataChannel>> sctp_data_channels()
-      const = 0;
+  // Call on the network thread to fetch stats for all the data channels.
+  // TODO(tommi): Make pure virtual after downstream updates.
+  virtual std::vector<DataChannel::Stats> GetDataChannelStats() const {
+    return {};
+  }
 
   virtual absl::optional<std::string> sctp_transport_name() const = 0;