Bug 875157 (part 1) - add isParentWindowMainWidgetVisible to nsIDOMWindowUtils.  r=bz

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extra : rebase_source : ba752abd8c16276e438d3af0742a9d74897acad0
Bug 875157 (part 2) - prevent background thumbnails from displaying alerts/dialogs.  r=adw

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extra : rebase_source : 972af12f9f6b35186826d6087df1e3d54a768367