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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