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}