This reverts commit 76daba3069.
We're going to need separate types for the JS-exposed style values, so
it doesn't make sense for us to match their names with our internal
types.
Previously we created a tree of CalculationNodes with dummy
UnparsedCalculationNode children, and then swapped those with the real
children. This matched the spec closely but had the unfortunate
downside that CalculationNodes couldn't be immutable, and couldn't know
their properties at construct-time. UnparsedCalculationNode is also a
footgun, as if it gets left in the tree accidentally we would VERIFY().
So instead, let's parse the calc() tree into an intermediate format, and
then convert each node in that tree, depth-first, into its
corresponding CalculationNode. This means each CalculationNode knows
what its children are when it is constructed, and they never change.
Apart from deleting UnparsedCalculationNode, we can also get rid of the
for_each_child_node() method that was only used by this "replace the
children" code.
The length resolution context might be needed even when resolving an
integer value, since it might contain a sign() function with length
values inside. This fixes a WPT subtest.