[Omnibox] Translate dropdown children instead of dropdown when animating

This keeps the background consistent during the animation, avoiding the
appearance of a "drop shadow" in the gap between the dropdown and the
toolbar.

Bug: 40933135
Change-Id: Iaf8308c8cc3ce31d5e49591e3ed2cf86a4ac1c1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422108


Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282728}