Remove dependency from RtpRtcp on the Module interface.

The 'Module' part of the implementation must not be
called via the RtpRtcp interface, but is rather a part of
the contract with ProcessThread. That in turn is an
implementation detail for how timers are currently implemented
in the default implementation.

Along the way I'm deprecating away the factory function which
was inside the interface and tied it to one specific implementation.
Instead, I'm moving that to the implementation itself and down the
line, we don't have to go through it if we just want to create an
instance of the class.

The key change is in rtp_rtcp.h and the new rtp_rtcp_interface.h
header file (things moved from rtp_rtcp.h), the rest falls from that.

Change-Id: I294f13e947b9e3e4e649400ee94a11a81e8071ce
Bug: webrtc:11581
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176419
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31440}
diff --git a/video/receive_statistics_proxy2.cc b/video/receive_statistics_proxy2.cc
index 15d08c4..3cce3c8 100644
--- a/video/receive_statistics_proxy2.cc
+++ b/video/receive_statistics_proxy2.cc
@@ -782,10 +782,10 @@
     return;
 
   if (!IsCurrentTaskQueueOrThread(worker_thread_)) {
-    // RtpRtcp::Configuration has a single RtcpPacketTypeCounterObserver and
-    // that same configuration may be used for both receiver and sender
-    // (see ModuleRtpRtcpImpl::ModuleRtpRtcpImpl).
-    // The RTCPSender implementation currently makes calls to this function on a
+    // RtpRtcpInterface::Configuration has a single
+    // RtcpPacketTypeCounterObserver and that same configuration may be used for
+    // both receiver and sender (see ModuleRtpRtcpImpl::ModuleRtpRtcpImpl). The
+    // RTCPSender implementation currently makes calls to this function on a
     // process thread whereas the RTCPReceiver implementation calls back on the
     // [main] worker thread.
     // So until the sender implementation has been updated, we work around this