Added missing @viewport to the simple UA stylesheet.

@viewport UA rule also required when the simple UA styles should otherwise
suffice to style the document.

BUG=298477

Review URL: https://chromiumcodereview.appspot.com/25092003

git-svn-id: svn://svn.chromium.org/blink/trunk@158488 bbb929c8-8fbe-4397-9dbb-9b2b20218538