Create the SrtpTransportInterface.

Create the SrtpTransportInterface, a subclass of RtpTransportInterface, which
allows the user to set the send and receive keys. The functionalities are
implemented inside the RtpTransportAdapters on top of BaseChannel.

BUG=webrtc:7013

Review-Url: https://codereview.webrtc.org/2714813004
Cr-Commit-Position: refs/heads/master@{#17023}
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index 716f8c9..3447e92 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -94,6 +94,7 @@
     "ortc/packettransportinterface.h",
     "ortc/rtptransportcontrollerinterface.h",
     "ortc/rtptransportinterface.h",
+    "ortc/srtptransportinterface.h",
     "ortc/udptransportinterface.h",
   ]