hide discardable factory from public imagegenerator api

BUG=skia:
R=halcanary@google.com, scroggo@google.com, djsollen@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/295243006

git-svn-id: http://skia.googlecode.com/svn/trunk/include@14889 2bbb7eff-a529-9590-31e7-b0007b416f81