layout_2020: Implement automatic minimum size of flex items
Implement the algorithm described in
https://drafts.csswg.org/css-flexbox/#min-size-auto.
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During layout it is often useful, for various specification reasons, to
know if an element is the `<body>` element of an `<html>` element root. There
are a couple places where a brittle heuristic is used to detect `<body>`
elements. This information is going to be even more important to
properly handle `<html>` elements that inherit their overflow property from
their `<body>` children.
Implementing this properly requires updating the DOM wrapper interface.
This check does reach up to the parent of thread-safe nodes, but this is
essentially the same kind of operation that `parent_style()` does, so is
ostensibly safe.
This change should not change any behavior and is just a preparation
step for properly handle `<body>` overflow.
This change adds support for the <iframe> element to Layout 2020. In
addition, certain aspects of the implementation are made the same
between both layout systems.
