We call attributeChangedCallback in two cases:
1. When any of the attributes in the observed attribute list has changed, appended, removed, or replaced.
2. When upgrading an element, for each attribute in element's attribute list that is in the observed attribute list.
Note: w/ Fixup for not implementing an API Enhancement Bug 1363481.
Tag UXP Issue #1344
1. It is possible that invoking a reaction triggers pushing a new ElementQueue into ReactionStack (e.g., calling define() in constructor which probably enqueue another upgrade reaction), and the reference of ElementQueue passed to InvokeReactions becomes invalid due to the memmove in nsTArray implementation.
2. And we get another benefit from this is memmove becomes faster.
Tag UXP Issue #1344
Bug 1347446 makes accessing ElementReactionQueue becomes a bit non-trival (have to get it via DocGroup). Since bug 1359346 already refactors the creation time of CustomElementData, ReactionQueue can also be put into CustomElementData, then we can just get ReactionQueue from Element.
Tag UXP Issue #1344
