Update API for Objective-C RTCPeerConnectionFactory. BUG= R=jiayl@webrtc.org, tkchin@webrtc.org Review URL: https://codereview.webrtc.org/1558473002 . Patch from Jon Hjelle <hjon@andyet.net>. Cr-Original-Commit-Position: refs/heads/master@{#11326} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: da99da81c928765ddf4f1ef17f13b9897c3d23b5