add serialprocs to MultiDocument, hide redundant methods

Bug: skia:
Change-Id: I6521e93af79439bd8c1d2f5130a68492044a2ee9
Reviewed-on: https://skia-review.googlesource.com/87788
Reviewed-by: Wei Li <weili@chromium.org>
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Reed <reed@google.com>
10 files changed