Cross-origin information should not be available via JavaScript.

We currently expose cross-origin information via the message we provide
for access check failures. We should use a sanitized message here rather
than exposing the information to JavaScript.

R=jochen@chromium.org
BUG=274408

Review URL: https://codereview.chromium.org/22889006

git-svn-id: svn://svn.chromium.org/blink/trunk@156217 bbb929c8-8fbe-4397-9dbb-9b2b20218538