Add option to SkDecodingImageGenerator to require unpremul.

Android needs to be able to set unpremul for purgeable
pixelRefs.

Once Android switches to the new interface, we can delete SkImageRef.

Bug=skia:2389

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

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/223903007

git-svn-id: http://skia.googlecode.com/svn/trunk@14084 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed