GitLab

Adds an observer method to notify update for reading list backend.

Bug: 1139133
Change-Id: Ic5483e075086d48334a9e347125048b5699c2560
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519151Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Brandon Wylie <wylieb@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825086}