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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tests/wpt/metadata-layout-2020/css/CSS2/floats-clear/clear-on-child-with-margins-2.html.ini

@@ -1,2 +0,0 @@
-[clear-on-child-with-margins-2.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/floats/adjoining-floats-dynamic.html.ini

@@ -1,2 +0,0 @@
-[adjoining-floats-dynamic.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/floats/floats-rule3-outside-left-002.xht.ini

@@ -1,2 +0,0 @@
-[floats-rule3-outside-left-002.xht]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/block_formatting_context_float_placement_a.html.ini

@@ -1,2 +0,0 @@
-[block_formatting_context_float_placement_a.html]
-  expected: FAIL