AudioDecoder: Remove the default implementation of SampleRateHz

And implement SampleRateHz in a bunch of mocks.

BUG=webrtc:5801
NOTRY=true

Review-Url: https://codereview.webrtc.org/2029543002
Cr-Original-Commit-Position: refs/heads/master@{#13161}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 347d35129ee61a4bce2fd732fdd2852e164257b2
6 files changed