[iOS] Fix Banner Promo View initialization order

The view controller sets the banner promo view's delegate from its own
bannerPromoDelegate in -loadView. However, with the other intialization
order, -loadView was called before the bannerPromoDelegate was set.

Fixed: 394570742
Change-Id: I4febd836b32c97ac08328d801041b973454b5ab7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6258914
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1420047}