[DevTools] UI improvements for responsive design view.

BUG=none
TBR=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/314303005

git-svn-id: svn://svn.chromium.org/blink/trunk@175667 bbb929c8-8fbe-4397-9dbb-9b2b20218538