Update to mandatory-to-implement statistics

[aboba]

Fix for Issue #833

Related WebRTC 1.-0 PR: w3c/webrtc-pc#1803

[aboba]

To allow ORTC to inherit mandatory-statistics from WebRTC 1.0, we can simplify the text as follows:

  The stats listed in [[!WEBRTC-STATS]] are intended to cover a wide
  range of use cases. Not all of them have to be implemented by every
  ORTC implementation.

  An ORTC implementation MUST support generating statistics of the types
  described in [[!WEBRTC10]] Section 8.6, with the exception of
  <code>RTCPeerConnectionStats</code>, when the corresponding
  objects exist, with the attributes that are listed when they are
  valid for that object. An implementation MAY support generating any
  other statistic defined in [[!WEBRTC-STATS]], and MAY generate
  statistics that are not documented.