SkPDF: clean up skpdfdocument, add static functions

Add SkPDFDocument::EmitPDF and SkPDFDocument::GetCountOfFontTypes

Also, make SkPDFDocument::appendPage return void, not bool.

Motivation: These static functions can be used to print subsets of a
pdf document (functionality to be added in a later CL).

BUG=skia:3585

Review URL: https://codereview.chromium.org/1036853002
2 files changed