Change SkDecodingImageGenerator to hide implementation details.

Motivation: A later CL will move SkDecodingImageGenerator into
include/ for Android to use.

R=scroggo@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/226253003

git-svn-id: http://skia.googlecode.com/svn/trunk@14066 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed