[CSS Grid Layout] Test coverage for first-line pseudo-element

According to the spec the ::first-line pseudo-element do not apply to
grid containers.

This was already working as expected but we were missing some tests
checking it.

TEST=fast/css-grid-layout/grid-container-ignore-first-line.html
TEST=fast/css-grid-layout/grid-item-first-line-valid.html

BUG=395788

Review URL: https://codereview.chromium.org/437363002

git-svn-id: svn://svn.chromium.org/blink/trunk@179640 bbb929c8-8fbe-4397-9dbb-9b2b20218538