Disable Local Echo for desktop capture

With audio support for desktop capture, we could have two use cases:
1. The sender and receiver are in the same room, then we hope to mute the
local playback during capturing, as the receiver speaker is nearby.
Example: google cast.

2. The sender and receiver are in different places, then we should make
both sides be able to hear the sound. Example: hangouts video chat.

Thus, we want to add a boolean field "disableLocalEcho" for audio
constraint of getDisplayMedia or getUserMedia, with this set to true, then
we mute the local playback during desktop capturing, otherwise unmute.

Any suggestions?