Move internal PeerConnection methods to PeerConnectionInternal

PeerConnectionInternal is being introduced so that it can be mocked in
tests and so that a fake can be written for it to be used by stats
tests.

Bug: webrtc:8764
Change-Id: I375d12ce352523e8ac584402685a7870bc399fac
Reviewed-on: https://webrtc-review.googlesource.com/43202
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21747}
diff --git a/pc/rtpsender.h b/pc/rtpsender.h
index 1fd7b37..092efe7 100644
--- a/pc/rtpsender.h
+++ b/pc/rtpsender.h
@@ -26,10 +26,11 @@
 #include "media/base/audiosource.h"
 #include "media/base/mediachannel.h"
 #include "pc/dtmfsender.h"
-#include "pc/statscollector.h"
 
 namespace webrtc {
 
+class StatsCollector;
+
 // Internal interface used by PeerConnection.
 class RtpSenderInternal : public RtpSenderInterface {
  public: