SkPDF: encode metadata strings correctly

BUG=skia:7669
Change-Id: I3a90a2406854cc9bcfdd299e09ae3d6e610f2cc7
Reviewed-on: https://skia-review.googlesource.com/111121
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
2 files changed