base: allows const reference args in ObserverList::Notify()

This CL updates ObserverList::Notify() to accept const reference
arguments (const arg&). Previously, Notify() rejects any move-only
arguments even if they are passed by const-reference.

This allows more flexible use of ObserverList with move-only types while
maintaining safety.

Bug: 40727208
Change-Id: Ice874bc689b19fa18b67ddf88d65865a4a9a29c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5878439
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1359185}