Reason for revert:
Seems to have broken Chromium tests.
Original issue's description:
> AcmReceiver: Look up last decoder in NetEq's table of decoders
>
> AcmReceiver::decoders_ is now one step closer to being unused.
>
> BUG=webrtc:5801
>
> Committed: https://crrev.com/1e4d8b574cde64d93b98d89c7b817fb93185a307
> Cr-Commit-Position: refs/heads/master@{#14274}
TBR=ossu@webrtc.org,henrik.lundin@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5801
Review-Url: https://codereview.webrtc.org/2348123002
Cr-Commit-Position: refs/heads/master@{#14279}
7a0f2c55f5