Remove WEBRTC_IOS from RTCPeerConnectionFactory public header.

We shouldn't make external users define this flag to use our file.

BUG=

Review URL: https://codereview.webrtc.org/1825713003

Cr-Original-Commit-Position: refs/heads/master@{#12106}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 24a62d5d83c5b5b58e77433a7373d42af5c6d565
3 files changed