I've written a PR for additional IceTransport read-only attributes and methods

It looks like the PR for DtlsTransport and IceTransport should be merged
soon.  Once it's in, I think it would be valuable to add a few additional
read-only attributes and methods that give the JS the ability to gain
information about the IceTransport, thing such as ICE role, candidates,
ufrag, and pwd.

In particular, I propose we add these:

IceTransport.role
IceTransport.component
IceTransport.gatheringstate
IceTransport.getLocalCandidates()
IceTransport.getRemoteCandidates()
IceTransport.getLocalParameters()  // ufrag + pwd
IceTransport.getRemoteParameters()  // ufrag + pwd
IceTransport.ongatheringstatechange


If you'd like to see the full PR, here it is:

https://github.com/w3c/webrtc-pc/pull/280