Introduce new runtime feature state "deprecated" and use it for showModalDialog

The state is an alias for testing, however, it makes it easier to see
that the feature is slated for removal, not launch.

BUG=345831
R=abarth@chromium.org,eseidel@chromium.org,jamesr@chromium.org,atwilson@chromium.org

Review URL: https://codereview.chromium.org/335453007

git-svn-id: svn://svn.chromium.org/blink/trunk@176540 bbb929c8-8fbe-4397-9dbb-9b2b20218538