Mask/background-repeat: round should round the number of tiles of an image to the nearest natural number greater than 0.

BUG=260691

Review URL: https://chromiumcodereview.appspot.com/24066007

git-svn-id: svn://svn.chromium.org/blink/trunk@158017 bbb929c8-8fbe-4397-9dbb-9b2b20218538