Render variant code in _finalize_params

This will be needed when we add support for variant grids. That's
because the _render function will render a whole grid of variants,
with a param holding the parameters for the whole grid. It will be too
late at this point to process indidivual variant with their own
individual params.

Bug: 330885075
Change-Id: I82923e262d238e280632cd40a8f5d2b602476cd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5373592


Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282139}