Update API for Objective-C RTCDataChannelConfiguration. BUG= Review URL: https://codereview.webrtc.org/1616363005 Cr-Original-Commit-Position: refs/heads/master@{#11405} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: da2183c86fcdf4cd525ea4cdc63bd3ba95703291