SkPDF: Fix Type3 ToUnicode table.

This seems to fix text extraction on Adobe Reader

  - Registry/Ordering is now set to Skia/SkiaOrdering.
  - Type3 fonts now get a FontDescriptor (force symbolic font).
  - CMapName is now Skia-Identity-SkiaOrdering
  - CMap behaves correctly for single-byte fonts.

Also:
  - SkTestTypeface returns tounicode map for testing.
  - Unit test updated

All PDFs render the same

BUG=skia:5606
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2292303004

Review-Url: https://codereview.chromium.org/2292303004
4 files changed