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}