Fix variations_safe_mode_egtest flaky tests.

Fixes the flakiness of the tests by disabling fetches of variations seed
from the server. When the code was built as chrome, it was fetching a
seed from the server and then loaded the experiments after restarting.
The code expected that no trials were set.

Forces the group of the fieldtrials experiment to be in control, since
the tests are still reading from / writing to Local State
(crbug.com/371171846).

Bug: 40073772
Change-Id: I70c500d18297736140c776cdd6223c5726327450
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6331553
Commit-Queue: Ramon Cano Aparicio <rcanoaparicio@google.com>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432205}