[conflict dialog] set #message element outline style to none

The browser's default style sheet :focus-visible outline rule will
draw an unwanted focus ring around the #message element.

Override the default style sheet: set the #message element outline
style to none.

Bug: b:255264604
Change-Id: If16b7eaf7b8e6992e8c9e4e66f38bd14d1381ca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4199117
Reviewed-by: Ben Reich <benreich@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1097728}