This makes the class concrete, and the former FrameBuffer3Proxy is the implementation.
Bug: webrtc:14003
Change-Id: Ife825b9f4efc7b79d9be8b4afb03904da819958a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/265868
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37793}
