Chromium implements AudioProcessorInterface::GetStats(), but other
clients may not. The existing stats were getting overwritten with
default AudioProcessorStats values in that case.
Now, we only overwrite the stats if the track has an
AudioProcessorInterface. Also, move signal level out of
SetAudioProcessingStats() to avoid the "don't set if it's -1" pattern.
Review URL: https://codereview.webrtc.org/1469803004
Cr-Commit-Position: refs/heads/master@{#10831}
2fe1cb0f0a