Reland "Refactor implementation of overflow indicator of UiElementContainerView"

This reverts commit aeeb7392fec20b2daba1ed942e264a02eda00c02.

Reason for revert: Reland with a fix.

Remove scroll bar variables from assistant_scroll_view as they are no
longer used. views::ScrollView always has scroll bar instances even if
scroll bar mode is set to kHiddenButEnabled, i.e. it's safe to call
ScrollView::vertical_scroll_bar.

Original change's description:
> Revert "Refactor implementation of overflow indicator of UiElementContainerView"
>
> This reverts commit 51ca7b0181eb21fe5ea816b922599be151497439.
>
> Reason for revert: Likely culprit for Assistant unittests failing in MSAN:
>
> https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Tests/24848/overview
>
> Original change's description:
> > Refactor implementation of overflow indicator of UiElementContainerView
> >
> > - Use more logic existing in ScrollView to determine when to show an
> >   overflow indicator.
> >
> > Bug: 1176919
> > Change-Id: Id92b2a426b4e0fea2e2a702d0146c2f4a88d167e
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3121225
> > Reviewed-by: Tao Wu <wutao@chromium.org>
> > Commit-Queue: Yuki Awano <yawano@google.com>
> > Cr-Commit-Position: refs/heads/main@{#919578}
>
> Bug: 1176919
> Change-Id: I2e2dc6d3edbb09af318dc73573c26556dbe6d88c
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3153002
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Commit-Queue: Tim Sergeant <tsergeant@chromium.org>
> Owners-Override: Tim Sergeant <tsergeant@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#920024}

Bug: 1176919
Change-Id: I00c7bd21e236f8e178d9ef9abebba196109b12bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3156765
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Yuki Awano <yawano@google.com>
Cr-Commit-Position: refs/heads/main@{#921037}