Reland "[responsive toolbar] Add menu separator"

This is a reland of commit 2ffbca84e82a85944f02c84635f0970fd5225586

Revert reason: ToolbarControllerUiTest::MenuSeparator fails on chrome
brand build since "Labs" button doesn't exist.
Fix: Remove ToolbarControlerUiTest::MenuSeparator. This test is fragile
because some buttons don't exist for particular OSes, e.g. brand build
has no "Labs" and ChromeOS Ash has no profile. Therefore it's hard to
observe separators given currently supported elements are few.

Original change's description:
> [responsive toolbar] Add menu separator
>
> Change-Id: I141626162b8500d22fa13dab161050b23d1f848a
> Bug: 1497494
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5050421
> Reviewed-by: Yuheng Huang <yuhengh@chromium.org>
> Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
> Commit-Queue: pengchao Cai <pengchaocai@chromium.org>
> Reviewed-by: Dana Fried <dfried@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1230055}

Bug: 1497494
Change-Id: I3fa1f7a80cf04f6af720019d6194088ca62dd84c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5069282
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: pengchao Cai <pengchaocai@chromium.org>
Reviewed-by: Yuheng Huang <yuhengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1230824}