Let AudioCodingModule::SendCodec return Maybe<CodecInst>

And deal with the consequences thereof...

Review URL: https://codereview.webrtc.org/1406123011

Cr-Original-Commit-Position: refs/heads/master@{#10497}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1fd4a4ab35f1fcb08b551befca180de9a1f55dfc
15 files changed