Remove DialogElement from ContextFeatures, remove plumbing from scripts.

The HTML <dialog> element is stable, and we don't have or need the ability
anymore to conditionally enable it for certain contexts. Remove the plumbing.

BUG=

Review URL: https://codereview.chromium.org/444563004

git-svn-id: svn://svn.chromium.org/blink/trunk@179639 bbb929c8-8fbe-4397-9dbb-9b2b20218538