Prepare StatsCollector to work with RtpTransceivers

This changes the StatsCollector to handle stats from multiple
MediaChannels of the same type (e.g., audio or video).

Bug: webrtc:8764
Change-Id: I91ba50d10cf469420189a311acdafbf6f78579b2
Reviewed-on: https://webrtc-review.googlesource.com/49560
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22009}
diff --git a/pc/peerconnection_bundle_unittest.cc b/pc/peerconnection_bundle_unittest.cc
index 116a598..616a076 100644
--- a/pc/peerconnection_bundle_unittest.cc
+++ b/pc/peerconnection_bundle_unittest.cc
@@ -76,8 +76,7 @@
   }
 
   cricket::VoiceChannel* voice_channel() {
-    auto transceivers =
-        GetInternalPeerConnection()->GetTransceiversForTesting();
+    auto transceivers = GetInternalPeerConnection()->GetTransceiversInternal();
     for (auto transceiver : transceivers) {
       if (transceiver->media_type() == cricket::MEDIA_TYPE_AUDIO) {
         return static_cast<cricket::VoiceChannel*>(
@@ -96,8 +95,7 @@
   }
 
   cricket::VideoChannel* video_channel() {
-    auto transceivers =
-        GetInternalPeerConnection()->GetTransceiversForTesting();
+    auto transceivers = GetInternalPeerConnection()->GetTransceiversInternal();
     for (auto transceiver : transceivers) {
       if (transceiver->media_type() == cricket::MEDIA_TYPE_VIDEO) {
         return static_cast<cricket::VideoChannel*>(