[DevTools] Combine media queries preview by sections.

MQs in one section now overlay. Hovering will show the values on the ruler.

BUG=409870

Review URL: https://codereview.chromium.org/526423002

git-svn-id: svn://svn.chromium.org/blink/trunk@181479 bbb929c8-8fbe-4397-9dbb-9b2b20218538