Add chromium support for MediaStreamAudioDestinationNode - part II

--- This CL is based on https://codereview.chromium.org/11369171/ ---

Add chromium support for MediaStreamAudioDestinationNode

BUG=none
TEST=add here

[henrika (OOO until Aug 14), 2012-12-28 09:46:20]

Chris,

I have uploaded a test version where I try to illustrate a way to improve this
CL. As we have discussed offline, we can't be dependent on microphone settings
when getUserMedia isn't even included. 

Patch #2 is an example on how we can use proper source settings depending on the
input from WebAudio. I don't know the best way to get these though? Can you
ensure one setting for all sources and match it well to WebRTC or must we do
further compensation in Chrome?

In any case, I don't feel that I can do more without your assistance to feed
down parameters for your source.

I still have an AP to figure out how to disable the AEC for this case. Guess the
proper way would be to use audio constraints but that is far from trivial.

To summarize: Chris, can you take the next step?

[henrika (OOO until Aug 14), 2012-12-28 09:52:22]

To test, use: 

http://www.corp.google.com/~henrika/WebAudio/MediaStreamAudioDestinationNode....

The AEC is still on => audio level is "pumping". Known issue.

Verify that microphone settings does not affect the end result.

[Chris Rogers, 2013-01-04 01:52:16]

Thanks Henrik, I was able to get your improved version working using your
simplified demo.  I've iterated this to include receiving format information
from WebKit (channels, sample-rate).

I've uploaded the latest version back in the original CL:
https://codereview.chromium.org/11369171/

Further comments there...

[henrika (OOO until Aug 14), 2013-01-04 10:29:15]

Great Chris, I will take a look when I am back on Monday!

Hope I did not complicate things bug uploading a second version. I assume we can
keep this CL as temp to exchange ideas until we are done.