Quoting golinux@??? (golinux@???):

> I just noticed that chromium has started spawning multiple PIDs that
> start with this string:
>
> PID XXXX chromium --type=renderer --field-trial-handle=1
> --primordial-pipe-token= and on and on including lots of number
> sequences.
[...]
> I searched a bit, and gather that this is phoning home with who
> knows what information about my habits. That can't be a good thing.
> Does anyone know how to kill it? Is there something comparable to
> about:config in chromium?

Possibly you've already seen http://raeknowler.com/wtf-chromium .
That doesn't have a solution, exactly, but does provide more detail.

Also,
https://groups.google.com/a/chromium.org/forum/#!msg/chromium-dev/6V-OhvCMxJE/EZR_jpPFCAAJ
says:

While I believe you cannot disable field trials altogether, you can at
least identify them by going to chrome://net-internals/#capture and
clicking on "Stop". The "Active Field Trial Groups" shows their names.
Then, I believe you can disable them one by one (or create a script that
takes the names and creates a command line disabling all of them) using
some command flag.

(Of course, in case it does not list all of the available (not
necessarily enabled) field trials, then some field trial may be added
the next time you launch Chrome. Wash, rinse, repeat)

'T'would be nice if the De??an package maintainer performed an
experimentecetomy.

Seems like there's an 'ungoogled-chromium' fork that has this covered:
https://github.com/Eloston/ungoogled-chromium/blob/master/patches/bromite/disable-fetching-field-trials.patch
https://github.com/Eloston/ungoogled-chromium

They claim to have functional builds for Stretch and Buster.