Whitespace string not valid as grid-template-area's value.

When entering a string with white-space only in the grid area definition, using
grid-template-area property, it's assumed as a row/column identifier.

The spec states cleary that empty strings should not produce
tokens, so the defintion has not a single token hence it should be
invalid.

BUG=412715

Review URL: https://codereview.chromium.org/558163004

git-svn-id: svn://svn.chromium.org/blink/trunk@182686 bbb929c8-8fbe-4397-9dbb-9b2b20218538