Mark some stats as only 'present' rather than 'valid' for some type o… (

#546)

* Mark some stats as only 'present' rather than 'valid' for some type of stream

Presumably the intent is that if the stream is not of the right kind, the data should not be included ('valid' makes it seem like it would be ok to send garbage data in other cases)

* Use member rather than attribute when talking about dictionary values

Also using "not present" for missing value, per WebIDL