SkPDF: store indirect references in object

Change-Id: Ia127247fd2d680dc6d06a04161bdf1da6701762e
Reviewed-on: https://skia-review.googlesource.com/c/170728
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
9 files changed