SkMultiPictureDocument : clean up MultiPictureDocumentReader interface

Motivation:  We may want to make SkMultiPictureDocument.h public in the
future.

Change-Id: Ie97b88d51a179c2283155d65bcadee32178115ca
Reviewed-on: https://skia-review.googlesource.com/11402
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Herb Derby <herb@google.com>
8 files changed