Removes inheritance and a virtual call. Also removes a root header that would have needed to be moved into a subdirectory otherwise to prevent circular dependencies. BUG=webrtc:4243 R=kjellander@webrtc.org, solenberg@webrtc.org TBR=mflodman@webrtc.org Review-Url: https://codereview.webrtc.org/1924793002 Cr-Commit-Position: refs/heads/master@{#12586}