Fix Chromium issue 503010263: forward focus traversal for focusable popover roots

## Chromium Issue

https://issues.chromium.org/issues/503010263

## Description

Fixed a bug where sequential focus traversal did not work properly with focusable popovers.

## Fix

Include scope root elements in forward traversal so invoker-opened focusable popovers are reachable via Tab.

## CL

https://chromium-review.googlesource.com/c/chromium/src/+/7768343

## Status

Merged