Add a test for painting only some grid items

This change adds a very simple visual test - we don't have any - that
checks that we properly paint partially visible scrolled grid
container. This means that we end up painting only some grid items.

This will act as a smoke-check for optimizing the painting code.

BUG=251446
TEST=fast/css-grid-layout/scrolled-grid-painting.html

Review URL: https://chromiumcodereview.appspot.com/19549009

git-svn-id: svn://svn.chromium.org/blink/trunk@154650 bbb929c8-8fbe-4397-9dbb-9b2b20218538