Const methods of base::ObserverList cannot be called concurrently on the same object in different threads. 

Especially, it is not safe to iterate over a const ObserverList, even using const iterators, simultaneously in multiple threads.

I don't know if this is an actual problem, but it is surprising.

The destructor of ObserverList::const_iterator decrements ObserverList::notify_depth_. This reference count is not atomic nor synchronized.

Note that the const_cast on https://cs.chromium.org/chromium/src/base/observer_list.h?l=191&rcl=c5dcb8488883447904af5b68e0aa9c1d68d56508 is somehow related to this issue.