-webkit-mask-repeat: round does not work

I have fixed this by adding a separate case for RoundFill repeat in RenderBoxModelObject.

BUG=chromium:260691

Review URL: https://chromiumcodereview.appspot.com/19785003

git-svn-id: svn://svn.chromium.org/blink/trunk@156461 bbb929c8-8fbe-4397-9dbb-9b2b20218538