Update image data upon changing the img.crossorigin attribute.

The spec tells us that setting the crossorigin attribute on an img
element should cause the "update the image data" steps to be invoked,

 http://www.whatwg.org/specs/web-apps/current-work/#update-the-image-data

Do so here, along with updating the underlying test's expectation.

R=
BUG=341047

Review URL: https://codereview.chromium.org/201143002

git-svn-id: svn://svn.chromium.org/blink/trunk@169382 bbb929c8-8fbe-4397-9dbb-9b2b20218538