This is a follow up of r174021 where we added support for placing grid items
using named grid lines. We left unfixed the case of the author using non
existent named grid lines. The truth is that it was already partially fixed
by some previous changes.

Instead of forcing resolveGridPositionFromStyle() to return a non resolved
grid position (like 'auto') we try to detect the usage of a non existent
named grid line before trying to resolve the grid position.

Review URL: https://codereview.chromium.org/309393002

git-svn-id: svn://svn.chromium.org/blink/trunk@176022 bbb929c8-8fbe-4397-9dbb-9b2b20218538