Implement CSS luminance masking using skia SkLumaMaskXfermode class.

BUG=289420

Review URL: https://chromiumcodereview.appspot.com/23947005

git-svn-id: svn://svn.chromium.org/blink/trunk@158106 bbb929c8-8fbe-4397-9dbb-9b2b20218538