Replace per-element OrderedHashMap storage for custom properties with
a RefCounted chain (CustomPropertyData) that enables structural
sharing. Each chain node stores only the properties declared directly
on its element, with a parent pointer to the inherited chain.
Elements that don't override any custom properties share the parent's
data directly (just a RefPtr copy). During cascade, only entries that
actually differ from the parent are stored in own_values - the rest
are inherited through the chain. During var() resolution, resolved
values are compared against the parent's and matching entries are
dropped, enabling further sharing.
The chain uses a depth limit (max 32) with flattening, plus
absorption of small parent nodes (threshold 8) to keep lookups fast.
This reduces custom property memory from ~79 MB to ~5.7 MB on
cloudflare.com.
StylePropertyMap only ever works with style properties - never
descriptors. Switching `has_property()` and `get_property_style_value()`
to taking PropertyNameAndID skips some duplicate work.
The limitations right now are:
- We don't know if a property is a list or not.
- We always reify as a CSSStyleValue directly.
So, we pass some tests but only ones that expect a CSSStyleValue.
After looking into this more, the `[[declarations]]` slot does not seem
to need to be a literal map of property names and values. Instead, it
can just point at the source (an element or style declaration), and
then direct all read or write operations to that.
This means the `has()` and `delete()` methods actually work now.
A few remaining failures in these tests are because of:
- StylePropertyMap[ReadOnly]s not being iterable yet
- We're not populating an element's custom properties map, which get
fixed whenever someone gets around to producing proper computed
values of them.
