Add ssid@ to page_content_annotations OWNERS

Working on page content cache, and I am also quite familiar with the
page contents annotation models processing.

Change-Id: Id86e3ca81e02beede317b7672ff10f7ef44a304c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7004305
Commit-Queue: Siddhartha S <ssid@chromium.org>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525162}