[ios] Fix fakebox pinning for landscape iphone

Previously when the Discover Feed was turned off, a miscalculation made
the NTP scrollview content not quite long enough to allow the fakebox
to be completely pinned, when an iphone was in landscape. This change
fixes that.

Video with fix:
http://screencast/cast/NTI3MDg3NDg1MTYzOTI5NnwxODFiZWQzOC03MA

Fixed: 335669702
Change-Id: Id63cec5bfeb87de8496b4aaf62190a1d15cf8499
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5462965


Reviewed-by: Adam Arcaro <adamta@google.com>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1289503}