Auto merge of #14130 - stshine:construct-flexbox, r=pcwalton

layout: Mark flex items properly during construction

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Set the flag of the fragment of children in a flex container according
to the direction of the container. The mark is done on the fragment
because flex item enstablish a stacking context when its z-index is
non-zero ,despite its `position' property.

Part of #14123.

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r? @pcwalton

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