SkPDF: consolidate two functions in SkPDFObjNumMap

Change-Id: Id815920e664ba986258867b7c35a7618a6927b0f
Reviewed-on: https://skia-review.googlesource.com/54360
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed