Note that we currently can't resolve calc() values without a layout
node, so when normalizing an image's source set, we'll flush any pending
layout updates and hope that gives us an up-to-date layout node.
I've left a FIXME about implementing this in a more elegant and less
layout-thrashy way, as that will require more architectural work.
This is intended as a replacement for Length and friends each holding a
`RefPtr<CalculatedStyleValue>`. Instead, let's make the types explicit
about whether they are calculated or not. This then means a Length is
always a Length, and won't require including `StyleValue.h`.
As noted, it's probably nicer for LengthOrCalculated to live in
`Length.h`, but we can't do that until Length stops including
`StyleValue.h`.
