RTCTransportStats.dtlsState replaces .activeConnection

In accordance with recent spec change:
https://github.com/w3c/webrtc-stats/pull/122

BUG=chromium:653873, chromium:627816

Review-Url: https://codereview.webrtc.org/2625993002
Cr-Original-Commit-Position: refs/heads/master@{#16098}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7064d5929a978aad8c3ce6cab61cef2c8c9203be
7 files changed