SkDocument:  consolidate trimBox code

Change-Id: I56f651320964eb3bd33475d21e6977803c4ca140
Reviewed-on: https://skia-review.googlesource.com/21370
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
7 files changed