With this change, the return value from NetEq::GetPlayoutTimestamp is
empty if the latest call to NetEq::GetAudio resulted in comfort noise
(codec-internal or external) being played out. This is because the
playout timestamp is not updated during CNG, and can therefore not be
trusted.
A few unit tests were updated to reflect the change.
BUG=webrtc:5669
Review URL: https://codereview.webrtc.org/1861303002
Cr-Commit-Position: refs/heads/master@{#12268}