clean up SkPicture include

Get SkPicture.h ready for documenting.

- remove private methods
- move private forward declarations
- name parameters

R=reed@google.com, bsalomon@google.com

Bug: skia:6898
Change-Id: I28829111203d8ae2a4661cf02c99023403aa0df0
Reviewed-on: https://skia-review.googlesource.com/134120
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@google.com>
13 files changed