[CSS Grid Layout] infinity should be defined as a negative value

This change makes us reject negative lengths at parse time, which
matches the updated specification, so that we can now safely assume
that when we resolve the length, it is not to a negative value.

BUG=234209
TESTS=fast/css-grid-layout/grid-columns-rows-get-set.html
      fast/css-grid-layout/grid-columns-rows-get-set-multiple.html

Review URL: https://chromiumcodereview.appspot.com/19668006

git-svn-id: svn://svn.chromium.org/blink/trunk@154805 bbb929c8-8fbe-4397-9dbb-9b2b20218538