Fix test failures caused by feature not being enabled.

The issue is caused by the feature not being explicitly enabled in
the browser tests. It instead relied on the features being enabled from
the `fieldtrial_testing_config.json` configuration, which is not used
in all test cases.

Change-Id: Ia5d19326a08e0e6c156f06ca905efa1bdac46681
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5429149


Commit-Queue: Liam Brady <lbrady@google.com>
Reviewed-by: Garrett Tanzer <gtanzer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283385}