AcmReceiver: use std::map instead of an array to keep the list of decoders

R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50419004

Cr-Original-Commit-Position: refs/heads/master@{#8824}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a4bef3e6c04e865e1a738286a76eac18ed0f24e8
2 files changed