use SkColorType instead of (deprecated) SkBitmap::Config

patch from issue 192423002

BUG=

Review URL: https://codereview.chromium.org/190543010

git-svn-id: svn://svn.chromium.org/blink/trunk@168857 bbb929c8-8fbe-4397-9dbb-9b2b20218538