Encapsulate all code related to variants into a _Variant class

Bug: 330885075
Change-Id: I39be91417fa7fb74886ddcc96563e503ba5af4fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5377646


Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282141}