Make "pixelated" equivalent to "auto" when downscaling.

This makes it consistent with the spec:
http://dev.w3.org/csswg/css-images/#the-image-rendering

Earlier CL adding pixelated:
http://crrev.com/336693005

BUG=386944

Review URL: https://codereview.chromium.org/352053002

git-svn-id: svn://svn.chromium.org/blink/trunk@176957 bbb929c8-8fbe-4397-9dbb-9b2b20218538