getVideoPlaybackQuality method is not available on HTMLAudioElement

@bbcrddave: 
"When using ```<audio>``` instead of ```<video>```, no video playback quality information is available so VideoModelExtensions.getPlaybackQuality would return null. The return value was always assumed to be good which would cause MetricsModel.addDroppedFrames to throw Uncaught TypeError: Cannot read property 'creationTime' of null.

This pr checks that getPlaybackQuality returns something useful and only calls addDroppedFrames if that is the case."