enabling `gn check` on the whole WebRTC repo

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2918803002
Cr-Original-Commit-Position: refs/heads/master@{#18390}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7d9a55b92db2f41b597a084e00853e017d94000f
2 files changed