use SkColorType instead of (deprecated) SkBitmap::Config for copyTo

TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/192373004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255918 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255918