Document default settings applied to tracks created by getUserMedia

[youennf]

As discussed in #775, it seems beneficial to document which defaults are selected by User Agents.
From a quick inspection, it seems that Chrome, Firefox and Safari agree on the following defaults:
video: 640x480, 30fps
audio: echoCancellation=true, noiseSuppression=true, autoGainControl=true

User Agents may use the same defaults, but at different point in the getUserMedia algorithm.
It might be worth exploring whether we can say something about this as well.

[youennf]

sampleRate and sampleByte could be on the side of using the OS defaults.

[youennf]

It might be worth exploring whether we can say something about this as well.
For this part, I was thinking we would not change how fitness distance is computed for device selection.
Once device is selected, we would select settings using the current algorithm based on the original constraints augmented with ideal values for all constraints that do not have ideal values. These added ideal values would be set to default values.