[DevTools] Remove unnecessary emulation checkboxes and simplify UX.

BUG=327641
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/340723005

git-svn-id: svn://svn.chromium.org/blink/trunk@176423 bbb929c8-8fbe-4397-9dbb-9b2b20218538