Now that the Rent-A-Codec no longer owns the encoders and decoders it
produces, they may outlive it. It's thus no longer correct for the
Rent-A-Codec to own the bandwidth estimation state; all of the
involved objects need to share ownership.
BUG=webrtc:5028
Review URL: https://codereview.webrtc.org/1821513003
Cr-Commit-Position: refs/heads/master@{#12159}