[webrtc-stats] Pull Request: Rename sender/receiver/track dictionaries

henbos has just submitted a new pull request for https://github.com/w3c/webrtc-stats:

== Rename sender/receiver/track dictionaries ==
Fixes #298.

Top-level rename (old => new):
```
RTCMediaStreamTrackStats => RTCMediaHandlerStats
```
Audio names:
```
RTCAudioStreamTrackStats => RTCAudioHandlerStats
RTCAudioSenderStats (not renamed)
RTCAudioReceiverStats (not renamed)
RTCLocalAudioStreamTrackStats => RTCSenderAudioTrackAttachmentStats
```
Video names:
```
RTCVideoStreamTrackStats => RTCVideoHandlerStats
RTCVideoSenderStats (not renamed)
RTCVideoReceiverStats (not renamed)
RTCLocalVideoStreamTrackStats => RTCSenderVideoTrackAttachmentStats
```

This forms the following hierarchy:
```
DICTIONARY NAME                          | INSTANCE FOUND BY
-----------------------------------------+--------------------------------------
RTCMediaHandlerStats                     | N/A
  RTCAudioHandlerStats                   | N/A
    RTCAudioSenderStats                  | kind == 'audio' && type == 'sender' 
      RTCSenderAudioTrackAttachmentStats | kind == 'audio' && type == 'track'
    RTCAudioReceiverStats                | kind == 'audio' && type == 'receiver'
  RTCVideoHandlerStats                   | N/A
    RTCVideoSenderStats                  | kind == 'video' && type == 'sender'
      RTCSenderVideoTrackAttachmentStats | kind == 'video' && type == 'track'
    RTCVideoReceiversStats               | kind == 'video' && type == 'receiver'
```
Where 'track' (attachment) stats have the same members as the senders but relative to the attachment period.

See https://github.com/w3c/webrtc-stats/pull/322