Turn some field trial debug instrumentation into a trial.
Turn some field trial debug instrumentation into a trial.

This is to diagnose what happened with crbug.com/359406,
where we saw the renderer crash disappear coinciding with
me landing https://codereview.chromium.org/1897493002 but
without any corresponding browser crashes appearing.

This change turns that instrumentation to be field-trial
based (50/50), so we can see if the renderer crashes
re-appear for the non-instrumentation group. The trial
is hard-coded because it needs to be initialized early
before variations trials are processed.

Also switching the debug token appending to not be
tied to the other instrumentation.

BUG=359406

Review-Url: https://codereview.chromium.org/1989353002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394609}

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