Update API for Objective-C RTCConfiguration.

BUG=

Review URL: https://codereview.webrtc.org/1616303002

Cr-Original-Commit-Position: refs/heads/master@{#11386}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6d49a8ed171c76fefb80f31629001c4c42f08996
10 files changed