[webrtc-pc] Merged Pull Request: Adding more detail to the definition of the ICE `disconnected` state.

alvestrand has just merged taylor-b's pull request 765 for 
https://github.com/w3c/webrtc-pc:

== Adding more detail to the definition of the ICE `disconnected` 
state. ==
This addresses issue #692, and follows the decision reached at the Aug
23 virtual iterim. It replaces "liveness checks have failed" with "the
ICE agent has determined that connectivity is currently lost". The way
this is determined is left implementation dependent; a couple examples
are provided.

See https://github.com/w3c/webrtc-pc/pull/765