Throw a SecurityError when accessing 'window.event' cross-origin.

Currently, we return 'undefined' when reading 'window.event' cross-origin
and ignore cross-origin writes. We should throw instead.

Gecko does not support 'window.event', so they don't throw here.

BUG=17325

Review URL: https://chromiumcodereview.appspot.com/22991005

git-svn-id: svn://svn.chromium.org/blink/trunk@156418 bbb929c8-8fbe-4397-9dbb-9b2b20218538