When using the SetEncoder interface, there's no actual CodecInst to return from Channel::GetSendCodec. Before this CL, this was done by calling the ACM, which has functionality for generating a CodecInst with the necessary values even when handed an external encoder. Unfortunately, this call takes a lock and does some extra processing which isn't strictly necessary in this case. Since GetSendCodec is called inside the audio input callback code, this can cause problems.
This CL instead generates a CodecInst in the SetEncoder call and has GetSendCodec simply return that one if it's available. If it isn't the value from codec_manager_ is returned instead, as was the case before injectable audio codec related changes were added to Channel.
BUG=b/38018041
Review-Url: https://codereview.webrtc.org/2924363004
Cr-Commit-Position: refs/heads/master@{#18515}
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