DevTools: explicitly control support for viewport meta tag so that desktop emulation was possible

BUG=314361
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/45363004

git-svn-id: svn://svn.chromium.org/blink/trunk@161228 bbb929c8-8fbe-4397-9dbb-9b2b20218538