Auto merge of #13820 - mrobinson:remove-nesting, r=glennw

Remove the concept of nested stacking contexts from display list builder

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With the removal of the legacy rendering path, we don't need to worry
about nested stacking contexts. This change just pushes the
complication to the WebRender conversion step, but a later change will
remove the concept from WebRender as well.

This also helps to prepare for the introduction of ids for particular
scrolling regions, an integral part of multiple ScrollLayers per block.

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