[Chrome Refresh] Update bookmarks bar separator

Updates color, size, shape, and margin.

Bug: 1399955
Change-Id: I08252833dd8d4be3d67f2425fd6b1c3b0b0aa372
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549505


Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Emily Shack <emshack@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146746}