[waffle]Fix 'More' button not scrolling to the bottom of the list

In some cases, the 'More' button didn't scroll to the bottom of the
choice list. This was due to the fact that the scroll position
that was set was not always enough.

We change this scroll position to the list's scrollHeight which will
make sure that clicking the 'More' button will always scroll to the
list's bottom.

Screen recording: http://b/314965772#comment2

Fixed: b:314965772
Change-Id: Icebb09db182de0371721417886a6b54027bffb71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5088747
Auto-Submit: Jack Yammine <jyammine@google.com>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Jack Yammine <jyammine@google.com>
Cr-Commit-Position: refs/heads/main@{#1233398}