Keep all other paddings when setting bottom paddings for NTP header

Before fix: https://screenshot.googleplex.com/4N28nXKJgm7V498
After fix: https://screenshot.googleplex.com/BPQkuPNqjTYqAk2

Bug: 340621486
Change-Id: Ia76dc15fbd08c572330a0b66b32244e1ccb1885d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5539470


Commit-Queue: Jian Li <jianli@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300877}