Add proto definition for the Masked Domain List.

Change-Id: I7785f3fa341c94f9b4c8a8b88bd3051c40a0733a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4400876


Commit-Queue: Alex Kallam <aakallam@chromium.org>
Reviewed-by: Theodore Olsauskas-Warren <sauski@google.com>
Reviewed-by: Steven Bingler <bingler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1128699}