[Chromoting] Implement down mixing in AudioPump

This change implements down mixing logic in AudioPump.

It adds 3 / 4 / 5 / 6 / 7 / 8 channels support in AudioPacket and down mixes the
packet into stereo before encoding. The newly added logic will only be executed
once multichannel output is returned by Windows API.

[email protected], [email protected]
BUG=669070

Review-Url: https://codereview.chromium.org/2903153004
Cr-Commit-Position: refs/heads/master@{#478488}
8 files changed