[iOS] Set clipsToBounds when changing collection view

Toggling the feed visibility or changing the selected feed changes
the main collection view of the NTPViewController, so we must set the
clipsToBounds property again.

Bug: 1292092
Change-Id: I2985d81035443cf8d86ca660e97abccbadd081a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3445622


Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Adam Arcaro <adamta@google.com>
Auto-Submit: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#968341}