Default streams: don't block media even if on different transceiver.

This fixes some edge cases where early media could cause default
stream that block the actual signaled media from beind delivered.

Bug: webrtc:11477
Change-Id: I8b26df63a690861bd19f083102d1395e882f8733
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183120
Commit-Queue: Taylor <deadbeef@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32030}

pc/channel_interface.h

@@ -52,6 +52,7 @@ class ChannelInterface {
   virtual bool SetRemoteContent(const MediaContentDescription* content,
                                 webrtc::SdpType type,
                                 std::string* error_desc) = 0;
+  virtual void SetPayloadTypeDemuxingEnabled(bool enabled) = 0;
 
   // Access to the local and remote streams that were set on the channel.
   virtual const std::vector<StreamParams>& local_streams() const = 0;