[Omnibox] Set clipToPadding to false for omnibox dropdown

This was the value of this attribute until recently; I am not exactly
sure where it regressed. Regardless, without it set to false there is a
small gap at the bottom of the list.

Bug: 343518012
Change-Id: I7eda1037f06cf6221b5f752ff28895ab90f101f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5582102
Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1307838}