Allow inserting padding into packets

[tonyherre]

UC2 requires the JS app to be able to pace sending, but doesn't allow any way to inject padding eg if the app wants to do some probing, but also when controlling packet sizes in general. We could inject data into the payload, but it would be better if there was a way to inject true rtp padding, with the padding RTP header bit set.

Suggested API:

dictionary RTCRtpPacketInit {
...
  // Causes padding bit to be set and padding added when serializing if > 0.
  unsigned long paddingBytes = 0;
}

and

interface RTCRtpPacket {
...
  readonly attribute unsigned long paddingBytes = 0;
}

CC @pthatcher @aboba @alvestrand