pdf_cg test expectations

The PNG files in this directory are the CoreGraphics rendering outputs for PDFs in //pdf/test/data/. They are generated from raw bitmaps using gfx::PNGCodec::Encode() using the |gfx::PNGCodec::FORMAT_BGRA| format. The PNGs are further optimized with optipng. Each PNG file is named after the PdfMetafileCgTest instance that uses it.