videoframechecker issues

[fippo]

not sure if I should file it here or in testrtc... I noticed some issues while trying to use it:

1. it uses the video elements width/height here
   which is not set for apprtc. This should be videoWidth/videoHeight, right?

2. it listens for the play event which means it can't be attached to video which already play:
   https://github.com/webrtc/testrtc/blob/master/src/js/camresolutionstest.js#L307
   I think the right thing is to listen for play only when the readyState is not 4/HAVE_ENOUGH_DATA and start checking right away when it is in state 4. @samdutton, you know more about this than I do :-)

cc @andresusanopinto