[iOS] [WebChannels] Maintain scroll position when switching feed

Prevents any jumps when switching feeds in the NTP. If not scrolled into
the feed, then maintains exact scroll position. If scrolled into the
feed, sets the scroll position to the top of the feed.

Bug: 1277974
Change-Id: Ief0087068f4d0265eb96b74faecc27654a5a5949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3601092


Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#995406}