Bookmarks++: Use proper context menu divider color

Bug: 1425477
Change-Id: I9e74e99ce4b7e4f8f0e14ce9de72c45e39bb2d48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4348478


Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Emily Shack <emshack@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118928}