SkPDF: maybe save some RAM by making the bitsets smaller

Wrap SkBitSet in a thin SkPDFGlyphUse class that manages new mapping.

Change-Id: Id97d42b8961f49c93fd45fdefad69d0aa9e273c5
Reviewed-on: https://skia-review.googlesource.com/c/163882
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
8 files changed