Implement a method to trigger an automatic scroll

When the user scrolls to the end of the Reading Mode side panel, the main page
will automatically scroll to the last line shown on the panel.

Screenshot of where the document scrolls to:
https://screenshot.googleplex.com/7x6CNiru7gCWtAp.png

Bug: 353590969
Change-Id: I05e0f64c67904e1188afa48d59773089f3618bd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5747514


Reviewed-by: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Sophia Tran <sophiatran@google.com>
Reviewed-by: Xiang Xiao <xiangxiao@google.com>
Cr-Commit-Position: refs/heads/main@{#1338808}