Clarify if stat object's members can change after having been returned

[stefhak]

From web perf group review [1]:

"Some pieces of the text hint that a Stat object’s members can be changed after it is returned. But an example shows 2 objects being compared to see how a baseline compares to a later “report”. Can stat object’s values be changed after they are returned?

http://www.w3.org/TR/2016/WD-webrtc-stats-20161214/#lifetime-of-stats-objects hints that an object that has been returned to JavaScript may change between accesses, but I could be misreading it."

[1] https://lists.w3.org/Archives/Public/public-web-perf/2016Dec/0014.html

[alvestrand]

Any such hint should be exterminated with extreme prejudice.
The intention of the stats object is that it is const after being created.

[dontcallmedom]

This was also reported by the original commenter in #136

[alvestrand]

Closing this as dup of #136, which is being addressed.