[CSS Grid Layout] Unknown grid area should compute to 'auto'

Per the specification, if the grid area specified for any <grid-line>
doesn't exist, it should compute to 'auto'. The original parsing
implementation allowed any <grid-line>, which is non-compliant.

This means that "grid-row: foobar / none;" is now (again) rejected
unless the grid element defines the required grid areas.

The testing framework was amended to reuse the existing grid element
so that we get the 'grid-template' definition as we assumed in the
testing. Also added a new test that checks that unknown grid areas
triggers the auto-placement algorithm when appropriate.

BUG=258092

Review URL: https://codereview.chromium.org/22353013

git-svn-id: svn://svn.chromium.org/blink/trunk@155850 bbb929c8-8fbe-4397-9dbb-9b2b20218538