[mediacapture-image] PhotoCapabilities::autoWhiteBalanceMode is a MediaSettingsItem, could be boolean

miguelao has just created a new issue for 
https://github.com/w3c/mediacapture-image:

== PhotoCapabilities::autoWhiteBalanceMode is a MediaSettingsItem, 
could be boolean ==
In [1] , ```autoWhiteBalanceMode``` is described as:
> This reflects whether automated White Balance Mode selection is on 
or off, and is boolean - on is true

So I would propose making it ```Boolean``` instead of 
```MediaSettingsItem```.

The equivalent ```autoWhiteBalanceMode``` setting in 
```PhotoSettings``` [2] is a boolean.

[1] http://w3c.github.io/mediacapture-image/index.html#attributes-4
[2] 
http://w3c.github.io/mediacapture-image/index.html#dictionary-photosettings-members

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-image/issues/14 using your GitHub 
account