[sensors] default sensor check during sensor construction

pozdnyakov has just created a new issue for 
https://github.com/w3c/sensors:

== default sensor check during sensor construction ==
As described in the 
[specification](https://w3c.github.io/sensors/#construct-sensor-object),
 default sensor needs to be returned at sensor construction if no 
sensor options were given by the user, and an exception should be 
raised if default sensor does not exist.

This requirement leads to a problem: whereas construction is 
synchronous web engine has to know the real sensors data beforehand, 
which unnecessarily complicates the implementation of the spec.


Please view or discuss this issue at 
https://github.com/w3c/sensors/issues/128 using your GitHub account