Auto merge of #7950 - mrobinson:layerize-iframes, r=pcwalton

Integrate iframes into the display list

Instead of always promoting iframes to StackingContexts, integrate them
into the display list. This prevents stacking bugs when
non-stacking-context elements should be drawn on top of iframes.

To accomplish this, we add another step to ordering layer creation,
where LayeredItems in the DisplayList are added to layers described by
the LayerInfo structures collected at the end of the DisplayList.
Unlayered items that follow these layered items are added to
synthesized layers.

Another result of this change is that iframe layers can be positioned
directly at the location of the iframe fragment, eliminating the need
for the SubpageLayerInfo struct entirely.

Iframes are the first type of content treated this way, but this change
opens up the possibility to properly order canvas and all other layered
content that does not create a stacking context.

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