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.
Manage containing blocks and WebRender SpaceAndClip during stacking
context tree constuction using the ContainingBlockInfo data structure.
This will allow us to reuse this data structure whenever we traverse the
fragment tree. In addition, StackingContextBuilder is no longer
necessary at all. This change also fixes some bugs where fixed position
fragments were not placed in the correct spatial node. Unfortunately,
these fixes are difficult to test because of #29659.
