[CSS Grid Layout] Add test coverage for Implicit Named Areas

Specs state in section 5.2.2 that explicitly adding named lines of the form
<foo-start>/<foo-end> should effectively create a named grid area (named foo
in this case). We got this for free in r174021 as a nice side effect of
implementing the resolution for named grid lines. We just lack proper test
coverage to ensure that we don't regress.

Review URL: https://codereview.chromium.org/313263003

git-svn-id: svn://svn.chromium.org/blink/trunk@176300 bbb929c8-8fbe-4397-9dbb-9b2b20218538