SkPDF: SkPDFFont organization changes.

SkPDFFont:
  - SkPDFType1Font::populate() encode advances correctly.
  - break out logically independent code into new files:
    * SkPDFConvertType1FontStream
    * SkPDFMakeToUnicodeCmap
    SkPDFFont.cpp is now 380 lines smaller.
    Expose `SkPDFAppendCmapSections()` for testing.

SkPDFFontImpl.h
  - Fold into SkPDFFont.

SkPDFConvertType1FontStream:
  - Now assume given a SkStreamAsset

SkPDFFont:
  - AdvanceMetric now hidden in a anonymous namespace.

No public API changes.
TBR=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2221163002

Review-Url: https://codereview.chromium.org/2221163002
10 files changed