Set permissions on experiments/field_trials.py

Since the previous policy was "anyone can add one", we're allowing
anyone to update this file for the time being.

Bug: webrtc:14154
Change-Id: Ib979797044f5eef014dce427ba6ad98837a98b46
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/321960
Reviewed-by: Emil Lundmark <lndmrk@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40838}
diff --git a/experiments/OWNERS b/experiments/OWNERS
new file mode 100644
index 0000000..116a653
--- /dev/null
+++ b/experiments/OWNERS
@@ -0,0 +1,2 @@
+# Allow anyone to update the field trial experiment list.
+per-file field_trials.py=*