Also introduce a typedef VideoTrackSourceInterface to be able to start changing clients such as Chrome to use the name VideoTrackSourceInterface. Document: https://docs.google.com/a/google.com/document/d/1mEIw_0uDzyHjL3l8a82WKp6AvgR8Tlwn9JGvhbUjVpY/edit?usp=sharing BUG=webrtc:5426 Review URL: https://codereview.webrtc.org/1758223002 Cr-Commit-Position: refs/heads/master@{#11854}