use SkImageInfo directly, instead of converting to SkBitmap::Config

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/128933004

git-svn-id: http://skia.googlecode.com/svn/trunk@12972 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed