Addresses: w3c/webrtc-pc#2930.

As an interface, RTCIceCandidatePair can be used as an attribute of
other interfaces.

So simplify RTCIceCandidatePairEvent by having a single candidatePair
attribute instead of individual local and remote candidates.

An application only acquires objects of RTCIceCandidatePair through
RTCIceTransport events and getSelectedCandidatePair().

So simplify input validation where RTCIceCandidatePair is used as an
argument by directly comparing object identity instead of using the
candidate pair match algorithm.

This ensures that each candidate pair has a unique identity, simplifying
the validation of RTCIceCandidatePair arguments to RTCIceTransport
methods.