[CSS Grid Layout] Replace <string> by <custom-ident> in grid-{area|column|row} syntax

Grid positioning properties no longer accept <string> values for
named grid lines as they did in previous versions of the specs. They
were first replaced by <ident> and since the beginning of 2014 they
should only accept <custom-ident>.

Updated the parser and the style resolver to this new syntax. Also
updated the tests accordingly.

Review URL: https://codereview.chromium.org/286793003

git-svn-id: svn://svn.chromium.org/blink/trunk@174313 bbb929c8-8fbe-4397-9dbb-9b2b20218538