Make CodecType conversion functions non-optional.

We can't handle no value here anyway and end up setting a default
at each call site. The defaults aren't even the same in each place.

BUG=None

Review-Url: https://codereview.webrtc.org/2998293002
Cr-Original-Commit-Position: refs/heads/master@{#19485}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1cdddc96fa2f81219bb363f540af8dfb822afbb3
19 files changed