Auto merge of #29757 - Loirooriol:sequential-context-2020, r=Loirooriol

Lay out floats and handle clearance in layout 2020, but don't flow text around them yet

This is a crude rebase of #27539

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components/layout_2020/flexbox/layout.rs

@@ -864,6 +864,7 @@ impl FlexLine<'_> {
                     flex_context.sides_to_flow_relative(item.padding),
                     flex_context.sides_to_flow_relative(item.border),
                     margin,
+                    Length::zero(),
                     collapsed_margin,
                 )
             })