Deletion of mutation observers from a list resulted in O(n^2) behavior and could lead to massive freezes.
This is resolved by using a LinkedList instead, reducing complexity to O(n).
A safely iterable doubly linked list was implemented based on `mozilla::DoublyLinkedList`,
allowing to insert and remove elements while iterating the list.
Due to the nature of `mozilla::DoublyLinkedList`, every Mutation Observer now inherits `mozilla::DoublyLinkedListElement<T>`.
This implies that a Mutation Observer can only be part of one DoublyLinkedList.
This conflicts with some Mutation Observers, which are being added to multiple `nsINode`s.
To continue supporting this, new MutationObserver base classes `nsMultiMutationObserver` and `nsStubMultiMutationObserver` are introduced,
which create `MutationObserverWrapper` objects each time they are added to a `nsINode`.
The wrapper objects forward every call to the actual observer.
Differential Revision: https://phabricator.services.mozilla.com/D157031
a88c7c233d