layout: Add LayoutBoxBase and use it for IndependentFormattingContext (#34507)

Add a new struct `LayoutBoxBase`, that will be used throughout the box
tree. The idea of this struct is that we have a place to consistently
store common layout information (style and node information) and also to
cache layout results such as content sizes (inline and maybe later box
sizes) and eventually layout results.

In addition to the addition of this struct,
`IndependentFormattingContext` is flattened slightly so that it directly
holds the contents of both replaced and non-replaced elements.

This is only added to independent formatting contexts, but will later be
added to all block containers as well.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout_2020/lib.rs

@@ -14,6 +14,7 @@ pub mod flow;
 mod formatting_contexts;
 mod fragment_tree;
 pub mod geom;
+mod layout_box_base;
 mod taffy;
 #[macro_use]
 pub mod layout_debug;