Add iceTransports to RTCConfiguration and add RTCOfferOptions for CreateOffer.

BUG=https://code.google.com/p/webrtc/issues/detail?id=3282

Review URL: https://codereview.chromium.org/361093002

git-svn-id: svn://svn.chromium.org/blink/trunk@178184 bbb929c8-8fbe-4397-9dbb-9b2b20218538