Make ACMCodecDB private to RentACodec

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1414203010

Cr-Original-Commit-Position: refs/heads/master@{#10549}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 39d8bee39725b6b3da70fab85c6a7e35b7052a06
7 files changed