SkPDF: pull out SkPDFSubsetFont code to another file.

Change-Id: Ib7f6b5a4e713beb537d1e0a8f20c7a8c0e666267
Reviewed-on: https://skia-review.googlesource.com/c/171226
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
4 files changed