Remove useThreadedHTMLParserForDataURLs settings.

This always defaults to true now.

BUG=308321,365509,370789

Review URL: https://codereview.chromium.org/292833002

git-svn-id: svn://svn.chromium.org/blink/trunk@174327 bbb929c8-8fbe-4397-9dbb-9b2b20218538