Auto merge of #8266 - mrobinson:stacking-context-mix, r=pcwalton

Mix stacking contexts into the positioned content list

Sometimes positioned content needs to be layered on top of stacking
contexts. The layer synthesis code can do this, but the current design
prevents it because stacking contexts are stored in a separate struct
member. In order to preserve tree order, mix stacking contexts into the
positioned content list, by adding a new StackingContextClass
DisplayItem. Such items do not have a base DisplayItem.

In some ways this simplifies the code, because we no longer have to
have a separate code path in the StackingContextLayerCreator.

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