Previously MessageReader and ProtobufMessageParse were passing done
callbacks to the messages handlers. These callbacks were necessary to
pace the reader, particularly when video renderer is slow and cannot
keep up with the rate of the incoming messages. It's no longer
necessary because we have explicit ACK messages for video packets.

Review URL: https://codereview.chromium.org/1655433002

Cr-Commit-Position: refs/heads/master@{#372509}