Merge 274068 "Always use the source channel layout with AudioBuf..."

Merge 274068 "Always use the source channel layout with AudioBuf..."

> Always use the source channel layout with AudioBufferConverter.
> 
> There are no web sites currently using this feature, but it causes
> all multichannel users to waste resources unnecessarily upmixing,
> generating, and transferring empty channel data.
> 
> Further this breaks fancy OSX multichannel users which have non
> standard channel layouts.  Mostly because we don't want to deal
> with the plethora of ways OSX wants to map channels ourselves.
> 
> Instead we can require that MSE users which want to change channel
> counts, must specify the maximum number of channels in the first
> initialization segment.
> 
> This also fixes use cases where clients routed multichannel streams
> into WebAudio for processing instead of actual playout.
> 
> BUG=266674
> TEST=Only 2 channels are created for stereo sources when a 7.1
> device is connected.
> 
> R=rileya@chromium.org, wolenetz@chromium.org
> 
> Review URL: https://codereview.chromium.org/304233006

TBR=dalecurtis@google.com

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274566

[DaleCurtis, 2014-06-03 17:41:40]

Committed patchset #1 manually as r274566.