Relax the field trial policy to not require an open bug

Having an associated bug with an owner should be sufficient information
for determining the status of a field trial. See the discussion starting
at [1] for more context.

[1] https://crbug.com/webrtc/14154#c11

Bug: webrtc:14154
No-Try: true
Change-Id: I7054ba25eac2af852a0d08770037938571e38921
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/321862
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Philipp Hancke <phancke@microsoft.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40837}