Add WebRuntimeFeatures::enableDialogElement

Chrome will use this function to enable <dialog> in WebUI as a trial
before exposing to the entire web.

Review URL: https://chromiumcodereview.appspot.com/22518003

git-svn-id: svn://svn.chromium.org/blink/trunk@156369 bbb929c8-8fbe-4397-9dbb-9b2b20218538