[Explore sites:] Fix scrolling so 'more' would bring up the 4th category, and scrolling would bring category to top.

Bug: 896044
Change-Id: Ifd000b49a08fb82ceae1f95a2552355143558f3c
Reviewed-on: https://chromium-review.googlesource.com/c/1285119
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: Cathy Li <chili@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#600198}(cherry picked from commit c00e16ab15a6375c0a639875d8f8caa8e8d5d688)
Reviewed-on: https://chromium-review.googlesource.com/c/1292449
Reviewed-by: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#165}
Cr-Branched-From: 4226ddf99103e493d7afb23a4c7902ee496108b6-refs/heads/master@{#599034}