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/test/fakepeerconnectionbase.h b/pc/test/fakepeerconnectionbase.h
index 790aa8f..b1cee6d 100644
--- a/pc/test/fakepeerconnectionbase.h
+++ b/pc/test/fakepeerconnectionbase.h
@@ -249,7 +249,7 @@
 
   std::vector<
       rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
-  GetTransceiversForTesting() const override {
+  GetTransceiversInternal() const override {
     return {};
   }