Support SVG element
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minimal SVG element implementation
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IFrame elements now manage FrameIds rather than the constellation.
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This PR stores the FrameId as well as the PipelineId in an html iframe. The iframes are now responsible for creating frame ids, not the constellation.
This is the first step in fixing #633, because it means we know the frame id of each script thread when it is created. It also means we can share the frame id, for example using it in the debugger.
cc @jdm, @ConnorGBrewster and @ejpbruel.
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