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
diff --git a/src/core/SkPictureRecord.h b/src/core/SkPictureRecord.h
index 0fdfaa7..a1b52fb 100644
--- a/src/core/SkPictureRecord.h
+++ b/src/core/SkPictureRecord.h
@@ -325,7 +325,7 @@
     bool     fOptsEnabled;
     int      fInitialSaveCount;
 
-    friend class SkPictureData;
+    friend class SkPictureData;   // for SkPictureData's SkPictureRecord-based constructor
     friend class SkPictureTester; // for unit testing
 
 #ifdef SK_COLLAPSE_MATRIX_CLIP_STATE