Have preloader handle crossorigin resources better.

Make the preloading of resources understand CORS better:

 - correctly interpret a 'crossorigin' attribute without a value
   as "anonymous".
 - enable a CORS fetch if the attribute was present. For <script>
   and <img>, currently.

R=
BUG=324974

Review URL: https://codereview.chromium.org/104943004

git-svn-id: svn://svn.chromium.org/blink/trunk@163210 bbb929c8-8fbe-4397-9dbb-9b2b20218538