GN: Add targets for AppRTCDemo on Mac+iOS

BUG=webrtc:6022
NOTRY=True

Review-Url: https://codereview.webrtc.org/2211413003
Cr-Original-Commit-Position: refs/heads/master@{#13666}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2ddfdba989df8e6ba767cfe856ba5c97cff527f7
7 files changed