UseCount deprecated WebKit-prefixed gradients

These were deprecated in the code in 2011 (when we introduced the
new syntax, see https://www.webkit.org/blog/1424/css3-gradients/),
but were never use counted.

Unfortunately no deprecation message as it's not simple to get hold
of the Console in the BisonCSSParser.

Review URL: https://codereview.chromium.org/136603006

git-svn-id: svn://svn.chromium.org/blink/trunk@166862 bbb929c8-8fbe-4397-9dbb-9b2b20218538