[iOS] Add property to PromoStyleVC to adjust scroll top constraint

This Cl adds a property to the PromoStyleViewController to allow
subclasses to constrain the top of the scroll view outside the
safeLayoutArea. This allows content to appear behind navigation bars.

CL with usage:
crrev.com/c/5416814

Screenshot with usage:
https://screenshot.googleplex.com/6VmiwRwB3MpmWJF

Bug: 326070899
Change-Id: I05570fa587f0998d3da3133e1faf923d7dcbb889
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5416713


Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281940}