OWNERS: Make everyone able to change *.gn,*.gni files.

Project-wide change to make it possible for all team members
to do changes to GN files.

NOTRY=True
R=kwiberg@webrtc.org
TBR=henrika@webrtc.org

Review URL: https://codereview.webrtc.org/2320043002 .

Cr-Original-Commit-Position: refs/heads/master@{#14163}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a41c13e6a2c472512095493e1755449eec309338
36 files changed