Datagram Transport Integration

- Implement datagram transport adaptor, which wraps datagram transport in DtlsTransportInternal. Datagram adaptor owns both ICE and Datagram Transports.
- Implement setup of datagram transport based on RTCConfiguration flag use_datagram_transport. This is very similar to MediaTransport setup with the exception that we create DTLS datagram adaptor.
- Propagate maximum datagram size to video encoder via MediaTransportConfig.

TODO: Currently this CL can only be tested in downstream projects. Once we add fake datagram transport, we will be able to implement unit tests similar to loopback media transport.

Bug: webrtc:9719
Change-Id: I4fa4a5725598dfee5da4f0f374269a7e289d48ed
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138100
Commit-Queue: Anton Sukhanov <sukhanov@webrtc.org>
Reviewed-by: Bjorn Mellem <mellem@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28047}
diff --git a/p2p/BUILD.gn b/p2p/BUILD.gn
index 5db66be..e404896 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -25,6 +25,8 @@
     "base/basic_packet_socket_factory.cc",
     "base/basic_packet_socket_factory.h",
     "base/candidate_pair_interface.h",
+    "base/datagram_dtls_adaptor.cc",
+    "base/datagram_dtls_adaptor.h",
     "base/dtls_transport.cc",
     "base/dtls_transport.h",
     "base/dtls_transport_internal.cc",