The purpose of this refactoring is a first step of separating the encoder parts from the RTP transport. BUG=webrtc:5687 R=mflodman@webrtc.org, pbos@webrtc.org, stefan@webrtc.org Review URL: https://codereview.webrtc.org/1864313003 . Cr-Commit-Position: refs/heads/master@{#12377}