Reland "Reland "Refactor SCTP data channels to use DataChannelTransportInterface.""

This is a reland of 487f9a17e426fd14bb06b13e861071b3f15d119b

Original change's description:
> Reland "Refactor SCTP data channels to use DataChannelTransportInterface."
> 
> Also clears SctpTransport before deleting JsepTransport.
> 
> SctpTransport is ref-counted, but the underlying transport is deleted when
> JsepTransport clears the rtp_dtls_transport.  This results in crashes when
> usrsctp attempts to send outgoing packets through a dangling pointer to the
> underlying transport.
> 
> Clearing SctpTransport before DtlsTransport removes the pointer to the
> underlying transport before it becomes invalid.
> 
> This fixes a crash in chromium's web platform tests (see
> https://chromium-review.googlesource.com/c/chromium/src/+/1776711).
> 
> Original change's description:
> > Refactor SCTP data channels to use DataChannelTransportInterface.
> >
> > This change moves SctpTransport to be owned by JsepTransport, which now
> > holds a DataChannelTransport implementation for SCTP when it is used for
> > data channels.
> >
> > This simplifies negotiation and fallback to SCTP.  Negotiation can now
> > use a composite DataChannelTransport, just as negotiation for RTP uses a
> > composite RTP transport.
> >
> > PeerConnection also has one fewer way it needs to manage data channels.
> > It now handles SCTP and datagram- or media-transport-based data channels
> > the same way.
> >
> > There are a few leaky abstractions left.  For example, PeerConnection
> > calls Start() on the SctpTransport at a particular point in negotiation,
> > but does not need to call this for other transports.  Similarly, PC
> > exposes an interface to the SCTP transport directly to the user; there
> > is no equivalent for other transports.
> 
> Bug: webrtc:9719
> Change-Id: I64e94b88afb119fdbf5f22750f88c8a084d53937
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151981
> Reviewed-by: Benjamin Wright <benwright@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Commit-Queue: Benjamin Wright <benwright@webrtc.org>
> Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29120}

Bug: webrtc:9719
Change-Id: I28481a3de64a3506bc57748106383eeba4ef205c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152740
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29290}
diff --git a/pc/sctp_utils.h b/pc/sctp_utils.h
index 468c960..6d41eb2 100644
--- a/pc/sctp_utils.h
+++ b/pc/sctp_utils.h
@@ -14,6 +14,8 @@
 #include <string>
 
 #include "api/data_channel_interface.h"
+#include "api/data_channel_transport_interface.h"
+#include "media/base/media_channel.h"
 
 namespace rtc {
 class CopyOnWriteBuffer;
@@ -36,6 +38,11 @@
                                  rtc::CopyOnWriteBuffer* payload);
 
 void WriteDataChannelOpenAckMessage(rtc::CopyOnWriteBuffer* payload);
+
+cricket::DataMessageType ToCricketDataMessageType(DataMessageType type);
+
+DataMessageType ToWebrtcDataMessageType(cricket::DataMessageType type);
+
 }  // namespace webrtc
 
 #endif  // PC_SCTP_UTILS_H_