*Scan: Explicitly disable ImmediateFreeing in fieldtrial_testing_config

This is needed to match presubmit internal checkers for the experiment.

Bug: 11297512
Change-Id: I770553d98b558d5a6ff7810a62cd6511bfa3ddc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2988677


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896458}