Add delegatesFocus check to tabindex blurs

Element::AttributeChanged will blur itself if its tabindex changes and
the element no longer supports focus. However, this check for supporting
focus does not consider delegatesFocus. This patch adds a check for a
child focusable element in the case that delegatesFocus is present.

Fixed: 1346606
Change-Id: I2757c9e4c103e98d3f32c8f73cdca9f7930c0d23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4182591
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1096629}