Revert r171889 "Make window.showModalDialog disabled at runtime"

This broke browser_tests ShowModalDialogTest.BasicTest.

TBR=eseidel@chromium.org
BUG=345831

Review URL: https://codereview.chromium.org/242683002

git-svn-id: svn://svn.chromium.org/blink/trunk@171921 bbb929c8-8fbe-4397-9dbb-9b2b20218538