CSSGradientValue should check whether gradientLength is zero or not.

BUG=295126
TEST=fast/gradients/css3-repeating-radial-gradients-crash.html

Review URL: https://chromiumcodereview.appspot.com/24350008

git-svn-id: svn://svn.chromium.org/blink/trunk@158220 bbb929c8-8fbe-4397-9dbb-9b2b20218538