[Device Posture] Attempt to fix Origin Trial test flakiness.

In some bots there is an average of ~9% flakiness where the
CSS testing fails out. While I couldn't reproduce it locally,
based on the log it is very likely that the issue is due to the
fact that the CSS code has not yet reacted to the posture changes
made in the initial part of the test. We can mitigate that by
making sure we receive the visual property changes before testing
the CSS values.

Bug: 339983706
Change-Id: I084b98a09557eb24102c41a524ffd714f2c62bff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5539889
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alexis Menard <alexis.menard@intel.com>
Cr-Commit-Position: refs/heads/main@{#1301240}