Move header negotiation state to transceivers.

The channel classes have stored the negotiated headers but a more
natural place to store them is in the RtpTransceiver class where
RtpHeaderExtension state is managed as well as the implementation of
the only method that depends on the stored state,
HeaderExtensionsNegotiated().

Also adding a TODO for further improvements where we're unnecessarily
storing state in the channel classes for the purposes of the transports.

Bug: webrtc:12726
Change-Id: If36668e3e49782ddeada23ebed126ee2c4935b8c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/216691
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33917}

pc/channel_interface.h

@@ -64,9 +64,6 @@ class ChannelInterface {
   //   * A DtlsSrtpTransport for DTLS-SRTP.
   virtual bool SetRtpTransport(webrtc::RtpTransportInternal* rtp_transport) = 0;
 
-  // Returns the last negotiated header extensions.
-  virtual RtpHeaderExtensions GetNegotiatedRtpHeaderExtensions() const = 0;
-
  protected:
   virtual ~ChannelInterface() = default;
 };