Also provide sample rate when registering decoders

This replaces the old practice of looking up the sample rate in a
table, which won't work when we add support for external decoders.

COAUTHOR=henrik.lundin@webrtc.org
BUG=4474
R=jmarusic@webrtc.org, minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/54469004

Cr-Commit-Position: refs/heads/master@{#9276}
10 files changed