[ios] Fix glitch when Fakebox transitions to pinned

This change fixes a miscalculation in heightAboveFeed that resulted in
a visual glitch when the user scrolled down and the Fakebox transitioned
to its pinned state. At the end of the transition, the Fakebox jumped up
to its final vertical position.

With the fix:
https://screenshot.googleplex.com/9Y7jM2eqs9WCL4g.png

Fixed: 440379980
Change-Id: I982f22e1391a9ea5202a59cea5b72720711e1bc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6872782
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1504749}