Throw an exception when calling 'window.showModalDialog' cross-origin.

This aligns our behavior with Gecko.

This patch also does a drive-by fix of the 'window.event' setter, which
was mistakenly omitted from https://codereview.chromium.org/22991005/.

BUG=17325

Review URL: https://chromiumcodereview.appspot.com/23185007

git-svn-id: svn://svn.chromium.org/blink/trunk@156463 bbb929c8-8fbe-4397-9dbb-9b2b20218538