Refactor webrtc/modules/audio_conference_mixer for GN check

This moves some GN check configurations out of .gn to individual
targets.

The now checked target is:
"//webrtc/modules/audio_conference_mixer/*"

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2593003002
Cr-Original-Commit-Position: refs/heads/master@{#15759}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 000d16396e748c7593ac41b24a876a6c20f0d277
1 file changed