Auto merge of #7423 - pcwalton:iframe-stacking-context-position, r=glennw

layout: Make the compositor rather than layout determine the position of each iframe.

The old code that attempted to do this during layout wasn't able to work
for multiple reasons: it couldn't know where the iframe was going to be
on the page (because of nested iframes), and at the time it was building
the display list for a fragment it couldn't know where that fragment was
going to be in page coordinates.

This patch rewrites that code so that only the size of an iframe is
determined during layout, and the position is determined by the
compositor. Layout layerizes iframes and marks the iframe layers with
the appropriate subpage ID so that the compositor can place them
correctly.

Closes #7377.

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