Fix CSSParserString::init crashing when given an empty UTF16 string.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/20149006

git-svn-id: svn://svn.chromium.org/blink/trunk@155007 bbb929c8-8fbe-4397-9dbb-9b2b20218538