...and setter. We had lots of places where we check if pseudo-element
type is specified and then use `pseudo_element_computed_properties()` or
`computed_properties()`. This change moves these checks from caller side
to the getter and setter.
There are multiple things happening here which are interconnected:
- We now use AbstractElement to refer to the source of a counter, which
means we also need to pass that around to compute `content`.
- Give AbstractElement new helper methods that are needed by
CountersSet, so it doesn't have to care whether it's dealing with a
true Element or PseudoElement.
- The CountersSet algorithms now walk the layout tree instead of DOM
tree, so TreeBuilder needs to wait until the layout node exists
before it resolves counters for it.
- Resolve counters when creating a pseudo-element's layout node. We
awkwardly compute the `content` value up to twice: Once to figure out
what kind of node we need to make, and then if it's a string, we do
so again after counters are resolved so we can get the true value of
any `counter()` functions. This will need adjusting in the future but
it works for now.
