[Editing] Fix text selection in focusable non-editable elements

Previously, SelectionHasFocus() would incorrectly return false when text
was selected within focusable but non-editable elements (e.g., elements
with tabindex="-1"). This occurred because the existing logic only
checked for ancestor/descendant relationships between the selection and
focused element, missing cases where they are siblings.

This change adds a new condition to SelectionHasFocus() that considers
the selection as focused when: 1. The focused element is explicitly
focusable but not editable 2. The selection's visible position matches
the focused element's position

This ensures that text selections in elements like <span tabindex="-1">
are properly displayed as focused, while maintaining existing behavior
for contenteditable elements and other focus scenarios.

Bug: 40664519
Change-Id: Ic858675ac74dd75edc8da13c77e3fb378001ca86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6721185
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Sambamurthy Bandaru <sambamurthy.bandaru@microsoft.com>
Commit-Queue: Tanu Jain <tanujain@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1489369}