Voe::Channel: Turned GetPlayoutFrequency into GetRtpTimestampRateHz.

This gets rid of a bit of codec-specific code in VoE.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2355483003
Cr-Original-Commit-Position: refs/heads/master@{#14614}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e280cdeb744c2e0c40fa1da24dce59436eda2e39
6 files changed