Prepare for introduction of rtc::PacketTransportInterface.

A rtc::PacketTransportInterface typedef is introduced to allow preparing
downstream projects for the upcoming refactoring of
cricket::Transport. This refactoring will introduce
rtc::PacketTransportInterface in https://codereview.webrtc.org/2416023002/ .

BUG=webrtc:6531

Review-Url: https://codereview.webrtc.org/2429803002
Cr-Commit-Position: refs/heads/master@{#14672}
diff --git a/webrtc/p2p/BUILD.gn b/webrtc/p2p/BUILD.gn
index 1b352f4..a41d8c0 100644
--- a/webrtc/p2p/BUILD.gn
+++ b/webrtc/p2p/BUILD.gn
@@ -36,6 +36,7 @@
     "base/p2ptransportchannel.cc",
     "base/p2ptransportchannel.h",
     "base/packetsocketfactory.h",
+    "base/packettransportinterface.h",
     "base/port.cc",
     "base/port.h",
     "base/portallocator.cc",