Add new video codec factories

This CL adds interfaces for the new video codec factories and wires them
up in WebRtcVideoEngine. The default behavior is unmodified however, and
the new code is currently unused except for the tests.

A follow-up CL will be uploaded for exposing them in the
PeerConnectionFactory API:


Review-Url: .
Cr-Original-Commit-Position: refs/heads/master@{#19828}
Cr-Mirrored-Commit: d4b0c0562323e24d3075b6831bafa62ea8bf32bd
8 files changed