[CSS Grid Layout] Fix GridLength::operator==

Fix a dumb mistake in the function where we would ignore
the type and the flex value for the purpose of equality.

BUG=235258

Review URL: https://chromiumcodereview.appspot.com/22825007

git-svn-id: svn://svn.chromium.org/blink/trunk@156127 bbb929c8-8fbe-4397-9dbb-9b2b20218538