This CL exposes the new type of video codec factories that represent all video codecs in the PeerConnectionFactory API, i.e. no extra internal SW video codecs will be added. Clients of the new functions will be responsible for adding all SW video codecs themselves, and also handling SW fallback and simulcast. BUG=webrtc:7925 R=deadbeef@webrtc.org Review-Url: https://codereview.webrtc.org/3004353002 . Cr-Commit-Position: refs/heads/master@{#19866}