Revert "[Omnibox] Translate dropdown children instead of dropdown when animating"

This reverts commit c58de9ea.

Reason for revert:
LUCI Bisection has identified this change as the culprit of a build failure. See the analysis: https://ci.chromium.org/ui/p/chromium/bisection/compile-analysis/b/8751552917183179281

Sample failed build: https://ci.chromium.org/b/8751552917183179281

If this is a false positive, please report it at http://b.corp.google.com/createIssue?component=1199205&description=Analysis%3A+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F5422108&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fci.chromium.org%2Fui%2Fp%2Fchromium%2Fbisection%2Fcompile-analysis%2Fb%2F8751552917183179281&type=BUG

Original change's description:
> [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}
>

Bug: 40933135
Change-Id: I8984d3e3faeb2a1b71904b18370587abb4b4c0f6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425823
Owners-Override: luci-bisection@appspot.gserviceaccount.com <luci-bisection@appspot.gserviceaccount.com>
Commit-Queue: luci-bisection@appspot.gserviceaccount.com <luci-bisection@appspot.gserviceaccount.com>
Bot-Commit: luci-bisection@appspot.gserviceaccount.com <luci-bisection@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1282743}