Auto merge of #25985 - mrobinson:inline-positioning-context, r=nox

layout_2020: Add support for hoisting positioned fragments in inline boxes

Add support for tracking containing blocks when doing inline layout.
This requires setting up a PositioningContext for inline boxes when
necessary. Instead of using the PositioningContext helper methods
and we reuse the contexts between line breaks.

Fixes #25279.

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- [x] These changes fix #25279

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