Delete ACMGenericCodec::Encode and use AudioEncoder::Encode directly

Move timestamp conversion out of ACMGenericCodec. Also remove lock
from ACMGenericCodec since the instance is always protected by
acm_crit_sect_ in AudioCodingModuleImpl.

Restructuring the code in AudioCodingModuleImpl::Encode to streamline
the use of locks.

R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46479004

Cr-Original-Commit-Position: refs/heads/master@{#8773}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fc562e0a56385ee9afa2ef0e5a5842798210cc23
6 files changed