[Compare] Add field trial config for compare

This includes both enabled and dogfood. This patch also includes
some updates to the ProductSpecificationButton which was managing
to acquire focus while being hidden (via some failing tests).

Bug: 364890646
Change-Id: I2687bf3d649d34678d93f94d7aa9b2780e3a4398
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5837801


Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Emily Shack <emshack@chromium.org>
Reviewed-by: Ankush Singh <ankushkush@google.com>
Cr-Commit-Position: refs/heads/main@{#1353296}