[iOS][OmniboxPosition] Fix hide header on tall content

Resolve the issue where the header becomes visible again after being
hidden.

One approach is to calculate the available space to verify if the header
has enough room in the view. However, this cannot be determined in
PromoStyleViewController, as the content view takes up all the available
space.
https://source.chromium.org/chromium/chromium/src/+/main:ios/chrome/common/ui/promo_style/promo_style_view_controller.mm;l=279-281;drc=98d3e3bfc351debfb2c36d4fb365d899b955776f

A simpler solution here is to prevent the header from reappearing after
it has been hidden.

Additionally, when hiding the header, also update the layout of the
scroll view to ensure that the dependent views (e.g., separators) are
updated correctly.

Bug: None
Change-Id: I7a19acabb6f8461ea304f1f089fe6310aeb2430e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5144972
Commit-Queue: Christian Xu <christianxu@chromium.org>
Auto-Submit: Christian Xu <christianxu@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1240194}