SkPDF: Don't forget to upcast before shift.

Change-Id: I3643749a7634e0b37f6e366e22190f09ac37f863
Reviewed-on: https://skia-review.googlesource.com/123044
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed