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/peerconnectioninternal.h b/pc/peerconnectioninternal.h
index 18a24b8..0f942d2 100644
--- a/pc/peerconnectioninternal.h
+++ b/pc/peerconnectioninternal.h
@@ -36,14 +36,13 @@
   // Returns true if we were the initial offerer.
   virtual bool initial_offerer() const = 0;
 
-  // TODO(steveanton): Remove these.
+  // TODO(steveanton): Remove these and replace with GetTransceiversInternal.
   virtual cricket::VoiceChannel* voice_channel() const = 0;
   virtual cricket::VideoChannel* video_channel() const = 0;
 
-  // Exposed for tests.
   virtual std::vector<
       rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
-  GetTransceiversForTesting() const = 0;
+  GetTransceiversInternal() const = 0;
 
   // Get the id used as a media stream track's "id" field from ssrc.
   virtual bool GetLocalTrackIdBySsrc(uint32_t ssrc, std::string* track_id) = 0;