Auto merge of #29870 - mrobinson:float-root, r=Loirooriol

Layout 2020: Correct rendering of floated root

Fix two issues around floating a root element:

1. In the StackingContext code handle the case where a root element is a Float fragment and not a Box fragment. This fixes a debug assertion failure in the css/CSS2/float/float-root.html test.
2. When initializing the SequentialLayoutState, use the containing block width as the maximum inline float placement position instead of infinity. This fixes the rendering of css/CSS2/float/float-root.html.

Note that css/CSS2/float/float-root.html was passing before, because both the test and reference were subject to the same bug. This fixes a couple other tests as well.

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components/layout_2020/flow/mod.rs

@@ -82,7 +82,7 @@ impl BlockFormattingContext {
         tree_rank: usize,
     ) -> IndependentLayout {
         let mut sequential_layout_state = if self.contains_floats || !layout_context.use_rayon {
-            Some(SequentialLayoutState::new())
+            Some(SequentialLayoutState::new(containing_block.inline_size))
         } else {
             None
         };