SkPDF: clean up fRasterDpi - PDFDevice no longer has a fRasterDpi; simply queries document. - #define DPI_FOR_RASTER_SCALE_ONE becomes constexpr float. - PDFShader::GetPDFShader no longer takes rasterScale or dpi - Remove un-needed factory functions. We're all adults here. Change-Id: Id2ce75d4e61af385763ccfb1db210465a1600067 Reviewed-on: https://skia-review.googlesource.com/21348 Reviewed-by: Ben Wagner <bungeman@google.com> Commit-Queue: Hal Canary <halcanary@google.com>