Sync: add proto visitor for ReadingList::estimated_read_time_seconds

This should've been added when that field was added to the proto [1]
but was overlooked at the time.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2957588

Bug: 1195978
Change-Id: Ibd3bc17aea1615ed537d0072f1a105b52119f8ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3122180


Commit-Queue: Marc Treib <treib@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915932}