This required quite a few small changes in the mixing algorithm
structure, the mixer interface and the mixer unit tests.
BUG=webrtc:6346
Review-Url: https://codereview.webrtc.org/2396483002
Cr-Commit-Position: refs/heads/master@{#14567}
The folder structure is now as was agreed on in the 'Slim and Modular
WebRTC' effort. Also added some dependencies that were previously in
another part of the tree.
NOTRY=True
Review-Url: https://codereview.webrtc.org/2238803002
Cr-Commit-Position: refs/heads/master@{#13742}
