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>
8 files changed