Side Panel: add dot indicator for when there are unseen items in the reading list.

Make the dot indicator match the size, placement, and color of the dot
indicator used for the chrome labs button. Reading list items added
while the side panel is open are automatically marked as seen.

Bug: 1248046
Change-Id: I0cbc95da34087f835d117818092751ceb035f7c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3158445


Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923330}