Bisected it down to this CL: https://codereview.chromium.org/294861300

I do the following:
- Start an appr.tc call between my MacBook Air and my workstation in one tab.
- Open YouTube in another tab (and let the first one keep running) on the MacBook.
- Mute the microphone in apprtc on my workstation.

At this point, the video feed from the MacBook (as shown on my workstation) stops updating and all the Chrome audio on the MacBook becomes terribly choppy. Looking at the logs, they're spewing out errors from different subsystems, like "UDP send of 1130 bytes failed with error 35", "AudioSyncReader::Read timed out, [...]" etc.

Sometimes I need to change back to apprtc and then back away from it for the problems to appear. They all disappear once I again focus the apprtc frame or unmute the mic on my workstation, but reappear once the mic is muted and apprtc is not the currently visible frame.

What is the expected result?
WebRTC continues to function even if not the currently active tab, regardless if the other side is sending audio or not.

What happens instead?
No video is sent from the affected computer. Other tabs' audio playout gets choppy.

I've only seen this happen on a MacBook Air, though I cannot confirm that it's limited to OSX.