Cleanup SkPicture* classes a bit

This is an attempt to reduce the number of friends the various SkPicture* classes have.
Probably the only controversial part is making getBitmap, getPath, getPicture and getPaint
public on SkPictureData (and adding a new initIterator entry point).

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

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/384753004
7 files changed