We need to:
a) Finalize the string format ( issue 672240  and  issue 667834 )
b) Determine whether profile 2 is actually supported before returning "probably" or true (MSE, EME).

As I've explained in CL https://codereview.chromium.org/2550413003/, we are probably better off supporting both the legacy vp9.2 codec id (since it's already used by YouTube) and whatever the new standard is.
And re #2 - we do already have a mechanism for overriding supported codecs per-platform via MediaClient, so I don't think that's a big issue.

While issue 477103 made it possible to do (b), it is difficult to ensure Chrome is doing the right thing. I filed  issue 672327  to improve this. I think we should address that before complicating the logic for VP9.

Issue 672337 tracks implementing the checks for Chrome.

Now that extended codec ids for VP9 have been standardized, this is obsolete